The high price of gem

Gemstone

A gemstone or gem is a piece of mineral, which, in cut and polished form, is used to make jewelry or other adornments...

-grade diamond

Diamond

In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

s, as well as significant ethical concerns of the diamond trade, have created a large demand for materials with similar gemological

Gemology

Gemology or gemmology is the science dealing with natural and artificial gems and gemstones. It is considered a geoscience and a branch of mineralogy...

 characteristics, known as diamond simulants or imitations. Simulants are distinct from synthetic diamond

Synthetic diamond

Synthetic diamond is diamond produced in a technological process; as opposed to natural diamond, which is created in geological processes. Synthetic diamond is also widely known as HPHT diamond or CVD diamond, denoting the production method, High-Pressure High-Temperature synthesis and Chemical...

, which unlike simulants is actual diamond, and therefore has the same material properties

Material properties of diamond

Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. Diamond is an optically isotropic crystal that is transparent to opaque. Owing to its strong covalent bonding, diamond is the hardest naturally occurring material...

 as natural diamond. Enhanced diamonds

Diamond enhancement

Diamond enhancements are specific treatments, performed on natural diamonds , which are designed to improve the gemological characteristics — and therefore the value — of the stone in one or more ways...

 are also excluded from this definition. A diamond simulant may be artificial, natural, or in some cases a combination thereof. While their material properties depart markedly from those of diamond, simulants have certain desired characteristics—such as dispersion

Dispersion (optics)

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...

 and hardness—which lend themselves to imitation. Trained gemologists with appropriate equipment are able to distinguish natural and synthetic diamonds from all diamond simulants, primarily by visual inspection.

The most common diamond simulants are high-leaded glass

Flint glass

Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00...

 (i.e., rhinestone

Rhinestone

A rhinestone or paste or diamante is a diamond simulant made from rock crystal, glass or acrylic.Originally, rhinestones were rock crystals gathered from the river Rhine. The availability was greatly increased around 1775 when the Alsatian jeweller Georg Friedrich Strass had the idea to imitate...

s) and cubic zirconia

Cubic zirconia

Cubic zirconia is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate...

 (CZ), both artificial materials. A number of other artificial materials, such as strontium titanate

Strontium titanate

Strontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure...

 and synthetic rutile

Rutile

Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...

 have been developed since the mid 1950s, but these are no longer in common use. Introduced at the end of the 20th century, the lab grown product moissanite

Moissanite

Moissanite originally referred to a rare mineral discovered by Henri Moissan having a chemical formula SiC and various crystalline polymorphs. Earlier, this material had been synthesized in the laboratory and named silicon carbide .- Background :...

 has gained popularity as an alternative to diamond.

Desired and differential properties

In order to be considered for use as a diamond simulant, a material must possess certain diamond-like properties. The most advanced artificial simulants have properties which closely approach diamond, but all simulants have one or more features that clearly and (for those familiar with diamond) easily differentiate them from diamond. To a gemologist, the most important of differential properties are those that foster non-destructive testing, and most of these are visual in nature. Non-destructive testing is preferred because most suspected diamonds are already cut into gemstones and set in jewelry, and if a destructive test (which mostly relies on the relative fragility and softness of non-diamonds) fails it may damage the simulant—this is not an acceptable outcome for most jewelry owners, as even if a stone is not a diamond it may still be of value.

Following are some of the properties by which diamond and its simulants can be compared and contrasted.

Durability and density

The Mohs scale of mineral hardness

Mohs scale of mineral hardness

The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...

 is a non-linear scale of common minerals' resistances to scratching. Diamond is at the top of this scale (hardness 10) as it is one of the hardest naturally-occurring materials known. (Some artificial substances, such as aggregated diamond nanorods

Aggregated diamond nanorods

Aggregated diamond nanorods, or ADNRs, are a nanocrystalline form of diamond, also known as "nanodiamond" or hyperdiamond. Nanodiamond was convincingly demonstrated to be produced by compression of graphite in 2003 and in the same work found to be much harder than bulk diamond, which makes it the...

, are harder.) Since diamonds are unlikely to encounter substances that can scratch it, other than another diamond, diamond gemstones are typically free of scratches. Diamond's hardness also is visually evident (under the microscope

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

 or loupe

Loupe

A loupe is a simple, small magnification device used to see small details more closely. Unlike a magnifying glass, a loupe does not have an attached handle, and its focusing lens are contained in an opaque cylinder or cone. Loupes are also called hand lenses .- Optics :Three basic types of loupes...

) by its highly lustrous

Lustre (mineralogy)

Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....

 facet

Facet

Facets are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure...

s (described as adamantine) which are perfectly flat, and its crisp, sharp facet edges. For a diamond simulant to be effective, it must be very hard relative to most gems. Most simulants fall far short of diamond's hardness, so they can be separated from diamond by their external flaws and poor polish.

In the recent past, the so-called "window pane test" was commonly thought to be an assured method of identifying diamond. It is a potentially destructive test wherein a suspect diamond gemstone is scraped against a pane of glass, with a positive result being a scratch on the glass and none on the gemstone. The use of hardness points and scratch plates made of corundum

Corundum

Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red...

 (hardness 9) are also used in place of glass. Hardness tests are inadvisable for three reasons: glass is fairly soft (typically 6 or below) and can be scratched by a large number of materials (including many simulants); diamond has four directions of perfect and easy cleavage

Cleavage (crystal)

Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...

 (planes of structural weakness along which the diamond could split) which could be triggered by the testing process; and many diamond-like gemstones (including older simulants) are valuable in their own right.

The specific gravity

Specific gravity

Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

 (SG) or density of a gem diamond is fairly constant at 3.52. Most simulants are far above or slightly below this value, which can make them easy to identify if unset. High-density liquids such as diiodomethane

Diiodomethane

Diiodomethane or methylene iodide, commonly abbreviated "MI", is a liquid organoiodine compound. It is insoluble in water, but soluble in ether and alcohol. It has a relatively high refractive index of 1.741, and a surface tension of 0.0508 N·m−1...

 can be used for this purpose, but they are all highly toxic

Toxin

A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

 so are usually avoided. A more practical method is to compare the expected size and weight of a suspect diamond to its measured parameters: for example, a cubic zirconia (SG 5.6–6) will be 1.7 times the expected weight of an equivalently sized diamond.

Optics and color

Diamonds are usually cut into brilliant

Brilliant (diamond cut)

A brilliant is a diamond or other gemstone, cut in a particular form with numerous facets so as to have exceptional brilliance. The shape resembles that of a cone and provides maximized light return through the top of the diamond....

s to bring out their brilliance, the amount of light reflected back to the viewer, and fire, the degree of colorful prismatic

Prism (optics)

In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...

 flashes seen. Both properties are strongly affected by the cut of the stone, but they are a function of diamond's high refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 (RI; the degree to which incident light is bent upon entering the stone) of 2.417 (as measured by sodium light, 589.3 nm) and high dispersion (the degree to which white light is split into its spectral color

Spectral color

A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths...

s within the stone) of 0.044, as measured by the sodium B and G line interval. Thus, if a diamond simulant's RI and dispersion are too low it will appear comparatively dull or "lifeless"; if the RI and dispersion are too high, the effect will be considered unreal or even tacky. Very few simulants have closely approximating RI and dispersion, but even the close simulants can be separated by an experienced observer. Direct measurements of RI and dispersion are impractical (a standard gemological refractometer

Refractometer

A refractometer is a laboratory or field device for the measurement of an index of refraction . The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone-Dale relation....

 has an upper limit of about RI 1.81), but several companies have devised reflectivity

Reflectivity

In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...

 meters to gauge a material's RI indirectly by measuring how well it reflects an infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 beam.

Perhaps equally as important is optic character. Diamond and other cubic (and also amorphous) materials are isotropic, meaning light entering a stone behaves the same way regardless of direction. Conversely, most minerals are anisotropic which produces birefringence

Birefringence

Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...

 or double refraction of light entering the material in all directions other than an optic axis

Optic axis of a crystal

The optic axis of a crystal is the direction in which a ray of transmitted light suffers no birefringence . Due to the internal structure of the crystal , light propagates along the optical axis differently than in other directions...

 (a direction of single refraction in a doubly refractive material). Under low magnification, this birefringence is usually detectable as a visual doubling of a cut gemstone's rear facets or internal flaws. An effective diamond simulant should therefore be isotropic.

Under longwave (365 nm) ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light, diamond may fluoresce

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce

Phosphorescence

Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum...

 yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet, in contrast to many diamond simulants. Similarly, because most diamond simulants are artificial they tend to have uniform properties: in a multi-stone diamond ring, one would expect the individual diamonds to fluoresce differently (in different colors and intensities, with some likely to be inert). If all the stones fluoresce in an identical manner, they are unlikely to be diamond.

Most "colorless" diamonds are actually tinted yellow or brown to some degree, whereas some artificial simulants are completely colorless—the equivalent of a perfect "D" in diamond color

Diamond color

A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or color. However, in reality almost no gem-sized natural diamonds are absolutely perfect. The color of a diamond may be affected by chemical impurities and/or structural defects in the crystal lattice...

 terminology. This "too good to be true" factor is important to consider; colored diamond simulants meant to imitate fancy diamonds are more difficult to spot in this regard, but the simulants' colors rarely approximate. In most diamonds (even colorless ones) a characteristic absorption spectrum can be seen (via a direct-vision spectroscope), consisting of a fine line at 415 nm. The dopant

Dopant

A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...

s used to impart color in artificial simulants may be detectable as a complex rare earth

Rare earth element

As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...

 absorption spectrum, which is never seen in diamond.

Also present in most diamonds are certain internal and external flaws or inclusions, the most common of which are fractures and solid foreign crystals. Artificial simulants are usually internally flawless, and any flaws that are present are characteristic of the manufacturing process. The inclusions seen in natural simulants will often be unlike those ever seen in diamond, most notably liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 "feather" inclusions. The diamond cutting

Diamond cutting

Diamond cutting is the art, skill and, increasingly, science of changing a diamond from a rough stone into a faceted gem. Cutting diamond requires specialized knowledge, tools, equipment, and techniques because of its extreme difficulty....

 process will often leave portions of the original crystal's surface intact. These are termed naturals and are usually on the girdle of the stone; they take the form of triangular, rectangular, or square pits (etch marks) and are seen only in diamond.

Thermal and electrical

Diamond is an extremely effective thermal conductor

Thermal conductivity

In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

 and usually an electrical

Electricity

Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...

 insulator

Electrical insulation

thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

. The former property is widely exploited in the use of an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistor

Thermistor

A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...

s mounted in a fine copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 tip. One thermistor functions as a heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

ing device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. As most simulants are thermal insulators, the thermistor's heat will not be conducted. This test takes about 2–3 seconds. The only possible exception is moissanite, which has a thermal conductivity similar to diamond: older probes can be fooled by moissanite, but newer Thermal and Electrical Conductivity testers are sophisticated enough to differentiate the two materials.
The latest development is nano diamond coating, an extremely thin layer of diamond material. If not tested properly it may show the same characteristics as a diamond. Diamond Testing.

A diamond's electrical conductance is only relevant to blue or gray-blue stones, because the interstitial boron

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

 responsible for their color also makes them semiconductor

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

s. Thus a suspected blue diamond can be affirmed if it completes an electric circuit successfully.

Artificial simulants

Diamond has been imitated by artificial materials for hundreds of years: advances in technology have seen the development of increasingly better simulants with properties ever nearer those of diamond. Although most of these simulants were characteristic of a certain time period, their large production volumes ensured that all continue to be encountered with varying frequency in jewelry of the present. Nearly all were first conceived for intended use in high technology, such as active laser medium

Active laser medium

The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...

s, varistor

Varistor

A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor...

s, and bubble memory

Bubble memory

Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data...

. Due to their limited present supply, collectors may pay a premium for the older types.

Summary table

Diamond simulants and their gemological properties

Material	Formula	Refractive index(es) Refractive index In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium.... 589.3 nm	Dispersion Dispersion (optics) In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media... 431 – 687 nm	Hardness (Mohs' scale Mohs scale of mineral hardness The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in... )	Density Specific gravity Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for... (g/cm3)	Thermal Cond. Thermal conductivity In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....	State of the art
Diamond Diamond In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...	C Carbon Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...	2.417	0.044	10	3.52	Excellent	1476 –
Artificial Simulants:
Glass Glass Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives... es	Silica with Pb Lead Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed... , Al, &/or Tl Thallium Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...	~ 1.6	> 0.020	< 6	2.4 – 4.2	Poor	1700 –
White Sapphire Sapphire Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...	Al2O3	1.762 – 1.770	0.018	9	3.97	Poor	1900–1947
Spinel Spinel Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...	MgO·Al2O3	1.727	0.020	8	~ 3.6	Poor	1920–1947
Rutile Rutile Rutile is a mineral composed primarily of titanium dioxide, TiO2.Rutile is the most common natural form of TiO2. Two rarer polymorphs of TiO2 are known:...	TiO2	2.62 – 2.9	0.33	~ 6	4.25	Poor	1947–1955
Strontium titanate Strontium titanate Strontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure...	SrTiO3	2.41	0.19	5.5	5.13	Poor	1955–1970
YAG	Y3Al5O12	1.83	0.028	8.25	4.55 – 4.65	Poor	1970–1975
GGG Gadolinium gallium garnet Gadolinium Gallium Garnet is a synthetic crystalline material of the garnet group, with good mechanical, thermal, and optical properties. It is typically colorless. It has cubic lattice, density 7.08 g/cm³ and Mohs hardness is variously noted as 6.5 and 7.5...	Gd3Ga5O12	1.97	0.045	7	7.02	Poor	1973–1975
Cubic Zirconia Cubic zirconia Cubic zirconia is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate...	ZrO2(+ rare earths)	~ 2.2	~ 0.06	~ 8.3	~ 5.7	Poor	1976 –
Moissanite Moissanite Moissanite originally referred to a rare mineral discovered by Henri Moissan having a chemical formula SiC and various crystalline polymorphs. Earlier, this material had been synthesized in the laboratory and named silicon carbide .- Background :...	SiC	2.648 – 2.691	0.104	8.5–9.25	3.2	High	1998 –
Natural Simulants:
Quartz Quartz Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...	Silica	1.543 – 1.554		7-	2.50 – 2.65		Ancient

The "refractive index(es)" column shows one refractive index for singly refractive substances, and a range for doubly refractive substances.

1700 onwards

The formulation of glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

es using lead

Lead

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

, alumina, and thallium

Thallium

Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...

 to increase RI and dispersion began in the late Baroque

Baroque

The Baroque is a period and the style that used exaggerated motion and clear, easily interpreted detail to produce drama, tension, exuberance, and grandeur in sculpture, painting, literature, dance, and music...

 period. These glasses are fashioned into brilliants, and when freshly cut they can be surprisingly effective diamond simulants. Known as rhinestones, pastes, or strass, glass simulants are a common feature of antique jewelry, and in such cases rhinestones can be valuable historical artifacts in their own right. The great softness (below hardnes 6) imparted by the lead means a rhinestone's facet edges and faces will quickly become rounded and scratched. Together with conchoidal fracture

Conchoidal fracture

Conchoidal fracture describes the way that brittle materials break when they do not follow any natural planes of separation. Materials that break in this way include flint and other fine-grained minerals, as well as most amorphous solids, such as obsidian and other types of glass.Conchoidal...

s, and air bubbles or flow lines within the stone, these features make glass imitations easy to spot under only moderate magnification. In contemporary production it is more common for glass to be molded rather than cut into shape: in these stones the facets will be concave and facet edges rounded, and mold marks or seams may also be present. Glass has also been combined with other materials to produce composites.

1900–1947

The first crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

line artificial diamond simulants were synthetic white sapphire

Sapphire

Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

 (Al

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

₂O₃, pure corundum) and spinel

Spinel

Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

 (MgO·Al₂O₃, pure magnesium

Magnesium

Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 aluminium oxide

Oxide

An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

). Both have been synthesized in large quantities since the first decade of the 20th century via the Verneuil

Verneuil process

The Verneuil process, also called flame fusion, was the first commercially successful method of manufacturing synthetic gemstones, developed in 1902 by the French chemist Auguste Verneuil. It is primarily used to produce the ruby and sapphire varieties of corundum, as well as the diamond simulants...

 or flame-fusion process, although spinel was not in wide use until the 1920s. The Verneuil process involves an inverted oxyhydrogen blowpipe

Blowpipe (tool)

The term blowpipe refers to one of several tools used to direct streams of gases into any of several working media.- Blowpipes for torches :...

, with purified feed powder mixed with oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 that is carefully fed through the blowpipe. The feed powder falls through the oxy-hydrogen flame, melts, and lands on a rotating and slowly descending pedestal below. The height of the pedestal is constantly adjusted to keep its top at the optimal position below the flame, and over a number of hours the molten powder cools and crystallizes to form a single pedunculated pear or boule

Boule (crystal)

A boule is a single-crystal ingot produced by synthetic means. A boule of silicon is the starting material for most of the integrated circuits used today....

 crystal. The process is an economical one, with crystals of up to 9 centimeters (3.5 inches) in diameter grown. Boules grown via the modern Czochralski process

Czochralski process

The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors , metals , salts, and synthetic gemstones...

 may weigh several kilograms.

Synthetic sapphire and spinel are durable materials (hardness 9 and 8) that take a good polish, but due to their much lower RI when compared to diamond (1.762–1.770 for sapphire, 1.727 for spinel) they are "lifeless" when cut. (Synthetic sapphire is also anisotropic, making it even easier to spot.) Their low RIs also mean a much lower dispersion (0.018 and 0.020), so even when cut into brilliants they lack the fire of diamond. Nevertheless synthetic spinel and sapphire were popular diamond simulants from the 1920s up until the late 1940s, when newer and better simulants began to appear. Both have also been combined with other materials to create composites. Commercial names once used for synthetic sapphire include Diamondette, Diamondite, Jourado Diamond, and Thrilliant. Names for synthetic spinel included Corundolite, Lustergem, Magalux, and Radiant.

1947–1970

The first of the optically "improved" simulants was synthetic rutile (TiO₂, pure titanium

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 oxide). Introduced in 1947–48, synthetic rutile possesses plenty of life when cut—perhaps too much life for a diamond simulant. Synthetic rutile's RI and dispersion (2.8 and 0.33) are so much higher than diamond that the resultant brilliants look almost opal

Opal

Opal is an amorphous form of silica related to quartz, a mineraloid form, not a mineral. 3% to 21% of the total weight is water, but the content is usually between 6% to 10%. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most...

-like in their display of prismatic colors. Synthetic rutile is also doubly refractive: although some stones are cut with the table perpendicular to the optic axis to hide this property, merely tilting the stone will reveal the doubled back facets.

The continued success of synthetic rutile was also hampered by the material's inescapable yellow tint, which producers were never able to remedy. However, synthetic rutile in a range of different colors, including blues and reds, were produced using various metal oxide dopants. These and the near-white stones were extremely popular if unreal stones. Synthetic rutile is also fairly soft (hardness ~6) and brittle, and therefore wears poorly. It is synthesized via a modification of the Verneuil process, which uses a third oxygen pipe to create a tricone burner—this is necessary to produce a single crystal, due to the much higher oxygen losses involved in the oxidation of titanium. The technique was invented by Charles H. Moore, Jr. at the South Amboy

South Amboy, New Jersey

South Amboy is a city in Middlesex County, New Jersey, on the Raritan Bay. As of the 2000 United States Census, the city population was 7,913.South Amboy, and Perth Amboy across the Raritan River, are collectively referred to as The Amboys...

, New Jersey

New Jersey

New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

-based National Lead Company (later N. L. Industries). National Lead and Union Carbide

Union Carbide

Union Carbide Corporation is a wholly owned subsidiary of The Dow Chemical Company. It currently employs more than 2,400 people. Union Carbide primarily produces chemicals and polymers that undergo one or more further conversions by customers before reaching consumers. Some are high-volume...

 were the primary producers of synthetic rutile, and peak annual production reached 750,000 carats (150 kg). Some of the many commercial names applied to synthetic rutile include: Astryl, Diamothyst, Gava or Java Gem, Meredith, Miridis, Rainbow Diamond, Rainbow Magic Diamond, Rutania, Titangem, Titania, and Ultamite.

National Lead was also where research into the synthesis of another titanium compound, strontium titanate (Sr

Strontium

Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...

TiO₃, pure tausonite), was conducted. Research was done during the late 1940s and early 1950s by Leon Merker and Langtry E. Lynd, who also used a tricone modification of the Verneuil process. Upon its commercial introduction in 1955, strontium titanate quickly replaced synthetic rutile as the most popular diamond simulant. This was due not only to strontium titanate's novelty, but to its superior optics: its RI (2.41) is very close to that of diamond, while its dispersion (0.19), although also very high, was a significant improvement over synthetic rutile's psychedelic display. Dopants were also used to give synthetic titanate a variety of colors, including yellow, orange to red, blue, and black. The material is also isotropic like diamond, meaning there is no distracting doubling of facets as seen in synthetic rutile.

Strontium titanate's only major drawback (if one excludes excess fire) is fragility. It is both softer (hardness 5.5) and more brittle than synthetic rutile—for this reason, strontium titanate was also combined with more durable materials to create composites. It was otherwise the best simulant around at the time, and at its peak annual production was 1.5 million carats (300 kg). Due to patent

Patent

A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

 coverage all US

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 production was by National Lead, while large amounts were produced overseas by Nakazumi Company of Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. Commercial names for strontium titanate included Brilliante, Diagem, Diamontina, Fabulite, and Marvelite.

1970–1976

From about 1970 strontium titanate began to be replaced by a new class of diamond imitations: the "synthetic garnet

Garnet

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...

s." These are not true garnets in the usual sense because they are oxides rather than silicate

Silicate

A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

s, but they do share natural garnet's crystal structure

Crystal structure

In mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...

 (both are cubic and therefore isotropic) and the general formula A₃B₂C₃O₁₂. While in natural garnets C is always silicon

Silicon

Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 and A and B may be one of several common element

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s, most synthetic garnets are composed of uncommon rare earth elements. They are the only diamond simulants (aside from rhinestones) with no known natural counterparts: gemologically they are best termed artificial rather than synthetic, because the latter term is reserved for human-made materials that can also be found in nature.

Although a number of artificial garnets were successfully grown, only two became important as diamond simulants. The first was yttrium aluminium garnet

Yttrium aluminium garnet

Yttrium aluminium garnet is a synthetic crystalline material of the garnet group. It is also one of three phases of the yttria-aluminium composite, the other two being yttrium aluminium monoclinic and yttrium aluminium perovskite . YAG is commonly used as a host material in various solid-state...

 (YAG; Y₃Al₅O₁₂) in the late 1960s. It was (and still is) produced via the Czochralski or crystal-pulling process, which involves growth from the melt. An iridium

Iridium

Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...

 crucible

Crucible

A crucible is a container used for metal, glass, and pigment production as well as a number of modern laboratory processes, which can withstand temperatures high enough to melt or otherwise alter its contents...

 surrounded by an inert

Inert

-Chemistry:In chemistry, the term inert is used to describe a substance that is not chemically reactive.The noble gases were previously known as inert gases because of their perceived lack of participation in any chemical reactions...

 atmosphere is used, wherein yttrium

Yttrium

Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...

 oxide and aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 oxide are melted and mixed together at a carefully controlled temperature of ca. 1980°C. A small seed crystal is attached to a rod which is lowered over the crucible until the crystal contacts the surface of the melted mixture. The seed crystal acts as a site of nucleation

Nucleation

Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

; the temperature is kept steady at a point where the surface of the mixture is just below the melting point. The rod is slowly and continuously rotated and retracted, and the pulled mixture crystallizes as it exits the crucible, forming a single crystal in the form of a cylindrical boule. The crystal's purity is extremely high, and it typically measures 5 cm (2 inches) in diameter and 20 cm (8 inches) long, and weighs 9,000 carats (1.75 kg).

YAG's hardness (8.25) and lack of brittleness were great improvements over strontium titanate, and although its RI (1.83) and dispersion (0.028) were fairly low, they were enough to give brilliant-cut YAGs perceptible fire and good brilliance (although still much lower than diamond). A number of different colors were also produced with the addition of dopants, including yellow, red, and a vivid green which was used to imitate emerald

Emerald

Emerald is a variety of the mineral beryl colored green by trace amounts of chromium and sometimes vanadium. Beryl has a hardness of 7.5–8 on the 10 point Mohs scale of mineral hardness...

. Major producers included ICT, INC. of Michigan, Litton Systems, Allied Chemical, Raytheon

Raytheon

Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...

, and Union Carbide; annual global production peaked at 40 million carats (8,000 kg) in 1972, but fell sharply thereafter. Commercial names for YAG included Diamonair, Diamonique, Gemonair, Replique, and Triamond.

While market saturation was one reason for the fall in YAG production levels, another was the recent introduction of the other artificial garnet important as a diamond simulant, gadolinium gallium garnet

Gadolinium gallium garnet

Gadolinium Gallium Garnet is a synthetic crystalline material of the garnet group, with good mechanical, thermal, and optical properties. It is typically colorless. It has cubic lattice, density 7.08 g/cm³ and Mohs hardness is variously noted as 6.5 and 7.5...

 (GGG; Gd₃Ga₅O₁₂). Produced in much the same manner as YAG (but with a lower melting point of 1750°C), GGG had an RI (1.97) close to, and a dispersion (0.045) nearly identical to diamond. GGG was also hard enough (hardness 7) and tough enough to be an effective gemstone, but its ingredients were also much more expensive than YAG's. Equally hindering was GGG's tendency to turn a dark brown upon exposure to sunlight

Sunlight

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

 or other ultraviolet source: this was due to the fact that most GGG gems were fashioned from impure material that was rejected for technological use. The SG of GGG (7.02) is also the highest of all diamond simulants and amongst the highest of all gemstones, which makes loose GGG gems easy to spot by comparing their dimensions with their expected and actual weights. Relative to its predecessors, GGG was never produced in significant quantities; it became more or less unheard of by the close of the 1970s. Commercial names for GGG included Diamonique II and Galliant.

1976 to present

Cubic zirconia

Cubic zirconia

Cubic zirconia is the cubic crystalline form of zirconium dioxide . The synthesized material is hard, optically flawless and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirconium silicate...

 or CZ (ZrO₂; zirconium dioxide

Zirconium dioxide

Zirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...

—not to be confused with zircon

Zircon

Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...

, a zirconium

Zirconium

Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

 silicate) quickly dominated the diamond simulant market following its introduction in 1976, and it remains the most gemologically and economically important simulant. CZ had been synthesized since 1930 but only in ceramic

Ceramic

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 form: the growth of single-crystal CZ would require an approach radically different from those used for previous simulants due to zirconium's extremely high melting point (2750°C), unsustainable by any crucible. The solution found involved a network of water-filled copper pipes and radio-frequency induction

Radio-frequency induction

Radio-frequency induction or RF induction is the use of a radio frequency magnetic field to transfer energy by means of electromagnetic induction in the near field...

 heating coils

Induction heating

Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...

; the latter to heat the zirconium feed powder, and the former to cool the exterior and maintain a retaining "skin" under 1 millimeter thick. CZ was thus grown in a crucible of itself, a technique called cold crucible (in reference to the cooling pipes) or skull crucible

Skull crucible

The Skull Crucible process was developed at the Lebedev Physical Institute in Moscow to manufacture cubic zirconia. Zirconium oxide powder is heated then gradually allowed to cool. Heating is accomplished by radio frequency induction using a coil wrapped around the apparatus...

 (in reference to either the shape of the crucible or of the crystals grown).

At standard pressure

Standard conditions for temperature and pressure

Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

 zirconium oxide would normally crystallize in the monoclinic rather than cubic crystal system: for cubic crystals to grow, a stabilizer must be used. This is usually Yttrium(III) oxide

Yttrium(III) oxide

Yttrium oxide is Y2O3. It is an air-stable, white solid substance. Yttrium oxide is used as a common starting material for both materials science as well as inorganic compounds.-Materials science:...

 or calcium oxide

Calcium oxide

Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

. The skull crucible technique was first developed in 1960s France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, but was perfected in the early 1970s by Soviet

Soviet Union

The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 scientists under V. V. Osiko at the Lebedev Physical Institute

Lebedev Physical Institute

The Lebedev Physics Institute of the Russian Academy of Sciences , situated in Moscow, is one of the leading Russian research institutes specializing in physics. It is also one of the oldest research institutions in Russia: its history dates back to a collection of physics equipment established by...

 in Moscow

Moscow

Moscow is the capital, the most populous city, and the most populous federal subject of Russia. The city is a major political, economic, cultural, scientific, religious, financial, educational, and transportation centre of Russia and the continent...

. By 1980 annual global production had reached 50 million carats (10,000 kg).

The hardness (8–8.5), RI (2.15–2.18, isotropic), dispersion (0.058–0.066), and low material cost make CZ the most popular simulant of diamond. Its optical and physical constants are however variable, owing to the different stabilizers used by different producers. It is important to realize that CZ is not a compound. There are many formulations of stabilized cubic zirconia. These variations change the physical and optical properties markedly. While the visual likeness of CZ is close enough to diamond to fool most who do not handle diamond regularly, CZ will usually give certain clues. For example: it is somewhat brittle and is soft enough to possess scratches after normal use in jewelry; it is usually internally flawless and completely colorless (whereas most diamonds have some internal imperfections and a yellow tint); its SG (5.6–6) is high; and its reaction under ultraviolet light is a distinctive beige. Most jewelers will use a thermal probe to test all suspected CZs, a test which relies on diamond's superlative thermal conductivity (CZ, like almost all other diamond simulants, is a thermal insulator). CZ is made in a number of different colors meant to imitate fancy diamonds (e.g., yellow to golden brown, orange, red to pink, green, and opaque black), but most of these do not approximate the real thing. Cubic zirconia can be coated with diamond-like carbon

Diamond-like carbon

Diamond-like carbon exists in seven different forms of amorphous carbon materials that display some of the typical properties of diamond. They are usually applied as coatings to other materials that could benefit from some of those properties. All seven contain significant amounts of sp3...

 to improve its durability, but will still be detected as CZ by a thermal probe.

CZ had virtually no competition until the 1998 introduction of moissanite (SiC; silicon carbide

Silicon carbide

Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...

). Moissanite is superior to cubic zirconia in two ways: its hardness (8.5–9.25) and low SG (3.2). The former property results in facets that are sometimes as crisp as a diamond's, while the latter property makes simulated moissanite somewhat harder to spot when unset (although still disparate enough to detect). However, unlike diamond and cubic zirconia, moissanite is strongly birefringent. This manifests as the same "drunken vision" effect seen in synthetic rutile, although to a lesser degree. All moissanite is cut with the table perpendicular to the optic axis in order to hide this property from above, but when viewed under magnification at only a slight tilt the doubling of facets (and any inclusions) is readily apparent.

The inclusions seen in moissanite are also characteristic: most will have fine, white, subparallel growth tubes or needles oriented perpedicular to the stone's table. It is conceivable that these growth tubes could be mistaken for laser drill holes that are sometimes seen in diamond (see diamond enhancement

Diamond enhancement

Diamond enhancements are specific treatments, performed on natural diamonds , which are designed to improve the gemological characteristics — and therefore the value — of the stone in one or more ways...

), but the tubes will be noticeably doubled in moissanite due to its birefringence. Like synthetic rutile, current moissanite production is also plagued by an as of yet inescapable tint, which is usually a brownish green. A limited range of fancy colors have been produced as well, the two most common being blue and green. Jewel-quality moissanite is produced by only one company, Charles & Colvard

Charles & Colvard

Charles & Colvard, Ltd., was founded in 1995 and is a publicly traded company based in the area of Research Triangle Park in North Carolina. It was the first company to produce and sell synthetic moissanite....

. Its limited availability makes moissanite about 120 times more expensive than cubic zirconia.

Natural simulants

Natural mineral

Mineral

A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s that (when cut) optically resemble white diamonds are rare, because the trace impurities usually present in natural minerals tend to impart color. The earliest simulants of diamond were colorless quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 (A form of silica, which also form obsidian

Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...

, glass

Glass

Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

 and sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

), crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 (a type of quartz), topaz

Topaz

Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO42. Topaz crystallizes in the orthorhombic system and its crystals are mostly prismatic terminated by pyramidal and other faces.-Color and varieties:...

, and beryl

Beryl

The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al26. The hexagonal crystals of beryl may be very small or range to several meters in size. Terminated crystals are relatively rare...

 (goshenite); they are all common minerals with above-average hardness (7–8), but all have low RIs and correspondingly low dispersions. Well-formed quartz crystals are sometimes offered as "diamonds," a popular example being the so-called "Herkimer diamond

Herkimer diamond

Herkimer diamond is a generic name for a double-terminated quartz crystal discovered within exposed outcrops of dolostone in and around Herkimer County, New York and the Mohawk River Valley...

s" mined in Herkimer County, New York

Herkimer County, New York

Herkimer County is a county located in the U.S. state of New York. It was created in 1791 north of the Mohawk River out of part of Montgomery County. As of the 2010 census, the population was 64,519. It is named after General Nicholas Herkimer, who died from battle wounds in 1777 after taking part...

. Topaz's SG (3.50–3.57) also falls within the range of diamond.

From a historical perspective, the most notable natural simulant of diamond is zircon. It is also fairly hard (7.5), but more importantly shows perceptible fire when cut, due to its high dispersion of 0.039. Colorless zircon has been mined in Sri Lanka

Sri Lanka

Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...

 for over 2,000 years; prior to the advent of modern mineralogy

Mineralogy

Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...

, colorless zircon was thought to be an inferior form of diamond. It was called "Matara diamond" after its source location. It is still encountered as a diamond simulant, but differentiation is easy due to zircon's anisotropy and strong birefringence (0.059). It is also notoriously brittle and often shows wear on the girdle and facet edges.

Much less common than colorless zircon is colorless scheelite

Scheelite

Scheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten. Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws...

. Its dispersion (0.026) is also high enough to mimic diamond, but although it is highly lustrous its hardness is much too low (4.5–5.5) to maintain a good polish. It is also anisotropic and fairly dense (SG 5.9–6.1). Synthetic scheelite produced via the Czochralski process is available, but it has never been widely used as a diamond simulant. Due to the scarcity of natural gem-quality scheelite, synthetic scheelite is much more likely to simulate it than diamond. A similar case is the orthorhombic carbonate

Carbonate

In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 cerussite

Cerussite

Cerussite is a mineral consisting of lead carbonate , and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruse to the mineral, whilst the present form, cerussite, is due to...

, which is so fragile (very brittle with four directions of good cleavage) and soft (hardness 3.5) that it is never seen set in jewelry, and only occasionally seen in gem collections because it is so difficult to cut. Cerussite gems have an adamantine luster, high RI (1.804–2.078), and high dispersion (0.051), making them attractive and valued collector's pieces. Aside from softness, they are easily distinguished by cerussite's high density (SG 6.51) and anisotropy with extreme birefringence (0.271).

Due to their rarity fancy-colored diamonds are also imitated, and zircon can serve this purpose too. Applying heat treatment to brown zircon can create several bright colors: these are most commonly sky-blue, golden yellow, and red. Blue zircon is very popular, but it is not necessarily color stable; prolonged exposure to ultraviolet light (including the UV component in sunlight) tends to bleach the stone. Heat treatment also imparts greater brittleness to zircon and characteristic inclusions.

Another fragile candidate mineral is sphalerite

Sphalerite

Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...

 (zinc blende). Gem-quality material is usually a strong yellow to honey brown, orange, red, or green; its very high RI (2.37) and dispersion (0.156) make for an extremely lustrous and fiery gem, and it is also isotropic. But here again, its low hardness (2.5–4) and perfect dodecahedral cleavage preclude sphalerite's wide use in jewelry. Two calcium-rich members of the garnet group fare much better: these are grossularite (usually brownish orange, rarely colorless, yellow, green, or pink) and andradite

Andradite

Andradite is a species of the garnet group. It is a nesosilicate, with formula Ca3Fe2Si3O12.Andradite includes three varieties:* Melanite: Black in color, referred to as "titanian andradite"....

. The latter is the rarest and most costly of the garnets, with three of its varieties—topazolite (yellow), melanite (black), and demantoid

Demantoid

Demantoid is the green gemstone variety of the mineral andradite, a member of the garnet group of minerals. Andradite is a calcium- and iron-rich garnet. The chemical formula is Ca3Fe23 with chromium substitution as the cause of the demantoid green color...

 (green)—sometimes seen in jewelry. Demantoid (literally "diamond-like") especially has been prized as a gemstone since its discovery in the Ural Mountains

Ural Mountains

The Ural Mountains , or simply the Urals, are a mountain range that runs approximately from north to south through western Russia, from the coast of the Arctic Ocean to the Ural River and northwestern Kazakhstan. Their eastern side is usually considered the natural boundary between Europe and Asia...

 in 1868; it is a noted feature of antique Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n and Art Nouveau

Art Nouveau

Art Nouveau is an international philosophy and style of art, architecture and applied art—especially the decorative arts—that were most popular during 1890–1910. The name "Art Nouveau" is French for "new art"...

 jewelry. Titanite

Titanite

Titanite, or sphene , is a calcium titanium nesosilicate mineral, CaTiSiO5. Trace impurities of iron and aluminium are typically present...

 or sphene is also seen in antique jewelry; it is typically some shade of chartreuse and has a luster, RI (1.885–2.050), and dispersion (0.051) high enough to be mistaken for diamond, yet it is anisotropic (a high birefringence of 0.105–0.135) and soft (hardness 5.5).

Discovered the 1960s, the rich green tsavorite

Tsavorite

Tsavorite or tsavolite is a variety of the garnet group species grossular, a calcium-aluminium garnet with the formula Ca3Al2Si3O12. Trace amounts of vanadium or chromium provide the green color....

 variety of grossular is also very popular. Both grossular and andradite are isotropic and have relatively high RIs (ca. 1.74 and 1.89, respectively) and high dispersions (0.027 and 0.057), with demantoid's exceeding diamond. However, both have a low hardness (6.5–7.5) and invariably possess inclusions atypical of diamond—the byssolite "horsetails" seen in demantoid are one striking example. Furthermore, most are very small, typically under 0.5 carats (100 mg) in weight. Their lusters range from vitreous to subadamantine, to almost metallic in the usually opaque melanite, which has been used to simulate black diamond. Some natural spinel is also a deep black and could serve this same purpose.

Composites

Because strontium titanate and glass are too soft to survive use as a ring stone, they have been used in the construction of composite or doublet diamond simulants. The two materials are used for the bottom portion (pavilion) of the stone, and in the case of strontium titanate, a much harder material—usually colorless synthetic spinel or sapphire—is used for the top half (crown). In glass doublets, the top portion is made of almandine

Almandine

Almandine , also known incorrectly as almandite, is a species of mineral belonging to the garnet Group. The name is a corruption of alabandicus, which is the name applied by Pliny the Elder to a stone found or worked at Alabanda, a town in Caria in Asia Minor. Almandine is an iron alumina garnet,...

 garnet; it is usually a very thin slice which does not modify the stone's overall body color. There have even been reports of diamond-on-diamond doublets, where a creative entrepreneur has used two small pieces of rough to create one larger stone.

In strontium titanate and diamond-based doublets, an epoxy

Epoxy

Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

is used to adhere the two halves together. The epoxy may fluoresce under UV light, and there may be residue on the stone's exterior. The garnet top of a glass doublet is physically fused to its base, but in it and the other doublet types there are usually flattened air bubbles seen at the junction of the two halves. A join line is also readily visible whose position is variable; it may be above or below the girdle, sometimes at an angle, but rarely along the girdle itself.

The most recent composite simulant involves combining a CZ core with an outer coating of laboratory created amorphous diamond. The concept effectively mimics the structure of a cultured pearl (which combines a core bead with an outer layer of pearl coating), only done for the diamond market.