Gemstone
Overview
 
A gemstone or gem is a piece of 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...

, which, in cut and polished form, is used to make jewelry
Jewellery
Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...

 or other adornments. However certain rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s, (such as lapis lazuli
Lapis lazuli
Lapis lazuli is a relatively rare semi-precious stone that has been prized since antiquity for its intense blue color....

) and organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 materials (such as amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...

 or jet
Jet (lignite)
Jet is a geological material and is considered to be a minor gemstone. Jet is not considered a true mineral, but rather a mineraloid as it has an organic origin, being derived from decaying wood under extreme pressure....

) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their lustre
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....

 or other physical properties that have aesthetic value.
Encyclopedia
A gemstone or gem is a piece of 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...

, which, in cut and polished form, is used to make jewelry
Jewellery
Jewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...

 or other adornments. However certain rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

s, (such as lapis lazuli
Lapis lazuli
Lapis lazuli is a relatively rare semi-precious stone that has been prized since antiquity for its intense blue color....

) and organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 materials (such as amber
Amber
Amber is fossilized tree resin , which has been appreciated for its color and natural beauty since Neolithic times. Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents...

 or jet
Jet (lignite)
Jet is a geological material and is considered to be a minor gemstone. Jet is not considered a true mineral, but rather a mineraloid as it has an organic origin, being derived from decaying wood under extreme pressure....

) are not minerals, but are still used for jewelry, and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their lustre
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....

 or other physical properties that have aesthetic value. Rarity is another characteristic that lends value to a gemstone. Apart from jewelry, from earliest antiquity until the 19th century engraved gems and hardstone carving
Hardstone carving
Hardstone carving is a general term in art history and archaeology for the carving for artistic purposes of semi-precious stones, also known as gemstones, such as jade, rock crystal , agate, onyx, jasper, serpentine or carnelian, and for an object made in this way. Normally the objects are small,...

s such as cups were major luxury art forms; the carvings of Carl Fabergé were the last significant works in this tradition.

Characteristics and classification

The traditional classification in the West, which goes back to the Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

s, begins with a distinction between precious and semi-precious stones; similar distinctions are made in other cultures. In modern usage the precious stones are 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...

, ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...

, 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...

 and 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...

, with all other gemstones being semi-precious. This distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond, and very hard, with hardnesses of 8–10 on the 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...

. Other stones are classified by their color, translucency and hardness. The traditional distinction does not necessarily reflect modern values, for example, while 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 are relatively inexpensive, a green garnet called 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....

, can be far more valuable than a mid-quality emerald. Another unscientific term for semi-precious gemstones used in art history
Art history
Art history has historically been understood as the academic study of objects of art in their historical development and stylistic contexts, i.e. genre, design, format, and style...

 and archaeology
Archaeology
Archaeology, or archeology , is the study of human society, primarily through the recovery and analysis of the material culture and environmental data that they have left behind, which includes artifacts, architecture, biofacts and cultural landscapes...

 is hardstone
Hardstone
Hardstone is a Kenyan musician. His music is a mixture of ragga, reggae and hiphop. He sings in English, Swahili and Kikuyu languages...

. Use of the terms 'precious' and 'semi-precious' in a commercial context is, arguably, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, which is not the case.

In modern times gemstones are identified by gemologists
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...

, who describe gems and their characteristics using technical terminology
Technical terminology
Technical terminology is the specialized vocabulary of any field, not just technical fields. The same is true of the synonyms technical terms, terms of art, shop talk and words of art, which do not necessarily refer to technology or art...

 specific to the field of gemology
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...

. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon
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...

 (C) and rubies of 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 (Al2O3). Next, many gems are crystals which are classified by their crystal system
Crystal system
In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...

 such as cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....

 or trigonal or monoclinic. Another term used is habit
Crystal habit
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates....

, the form the gem is usually found in. For example diamonds, which have a cubic crystal system, are often found as octahedrons.

Gemstones are classified into different groups, species, and varieties. For example, ruby is the red variety of the species 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...

, while any other color of corundum is considered sapphire. 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...

 (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species 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...

.

Gems are characterized in terms of 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....

, 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...

, 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...

, 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...

, 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...

, fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

, and luster. They may exhibit pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...

 or double refraction. They may have luminescence
Luminescence
Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions, or stress on a crystal. This distinguishes luminescence from incandescence, which is light emitted by a...

 and a distinctive absorption spectrum.

Material or flaws within a stone may be present as inclusions
Inclusion (mineral)
In mineralogy, an inclusion is any material that is trapped inside a mineral during its formation.In gemology, an inclusion is a characteristic enclosed within a gemstone, or reaching its surface from the interior....

.

Gemstones may also be classified in terms of their "water". This is a recognized grading of the gem's luster and/or transparency and/or "brilliance". Very transparent gems are considered "first water
First Water
First Water means "highest quality" and is a term which originates from the gemstone trade. The clarity of diamonds is assessed by their translucence; the more like water, the higher the quality. This comparison of diamonds with water dates back to at least the early 17th century, and Shakespeare...

", while "second" or "third water" gems are those of a lesser transparency.

Value

There is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by the Gemological Institute of America
Gemological Institute of America
The Gemological Institute of America, or GIA, is a nonprofit institute dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect all buyers and sellers of gemstones by setting and maintaining the standards used to evaluate...

 (GIA) in the early 1950s. Historically, all gemstones were graded using the naked eye. The GIA system included a major innovation: the introduction of 10x magnification as the standard for grading clarity. Other gemstones are still graded using the naked eye (assuming 20/20 vision).

A mnemonic device, the "four Cs" (color, cut, clarity and carats), has been introduced to help the consumer understand the factors used to grade a diamond. With modification, these categories can be useful in understanding the grading of all gemstones. The four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamond. In diamonds, cut is the primary determinant of value, followed by clarity and color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors (dispersion), chop it up into bright little pieces (scintillation), and deliver it to the eye (brilliance). In its rough crystalline form, a diamond will do none of these things; it requires proper fashioning and this is called "cut". In gemstones that have color, including colored diamonds, it is the purity and beauty of that color that is the primary determinant of quality.

Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent (emeralds will always have a number of inclusions), cut, unusual optical phenomena within the stone such as color zoning, and asteria (star effects). Color zoning is the uneven distribution of coloring within a gem. The Greeks, for example, greatly valued asteria in gemstones, which were regarded as a powerful love charm, and Helen of Troy was known to have worn star-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...

.

Historically, gemstones were classified into precious stones and semi-precious stones. Because such a definition can change over time and vary with culture, it has always been a difficult matter to determine what constitutes precious stones.

Aside from the 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...

, the ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...

, 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...

, 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...

, pearl
Pearl
A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...

 (strictly speaking not a gemstone) and 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...

 have also been considered to be precious. Up to the discoveries of bulk amethyst
Amethyst
Amethyst is a violet variety of quartz often used in jewelry. The name comes from the Ancient Greek ἀ a- and μέθυστος methustos , a reference to the belief that the stone protected its owner from drunkenness; the ancient Greeks and Romans wore amethyst and made drinking vessels of it in the belief...

 in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. Even in the last century certain stones such as aquamarine, peridot
Peridot
-Chemistry:The chemical composition of peridot is 2SiO4, with Mg in greater quantities than Fe.-Etymology:The origin of the name "peridot" is uncertain...

 and cat's eye have been popular and hence been regarded as precious.

Nowadays such a distinction is no longer made by the trade. Many gemstones are used in even the most expensive jewelry, depending on the brand name of the designer, fashion trends, market supply, treatments, etc. Nevertheless, diamonds, rubies, sapphires and emeralds still have a reputation that exceeds those of other gemstones.

Rare or unusual gemstones, generally meant to include those gemstones which occur so infrequently in gem quality that they are scarcely known except to connoisseurs, include andalusite
Andalusite
Andalusite is an aluminium nesosilicate mineral with the chemical formula Al2SiO5.The variety chiastolite commonly contains dark inclusions of carbon or clay which form a checker-board pattern when shown in cross-section....

, axinite
Axinite
Axinite is a brown to violet-brown, or reddish-brown bladed group of minerals composed of calcium aluminium boro-silicate, 3Al2BO3Si4O12OH...

, cassiterite
Cassiterite
Cassiterite is a tin oxide mineral, SnO2. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem...

, clinohumite
Clinohumite
Clinohumite is an uncommon member of the humite group, a magnesium silicate according to the chemical formula 942. Most commonly found as tiny indistinct grains, large euhedral clinohumite crystals are sought by collectors and occasionally fashioned into bright, yellow-orange gemstones...

 and red beryl.

Gem prices can fluctuate heavily (such as those of tanzanite
Tanzanite
Tanzanite is the blue/purple variety of the mineral zoisite which was discovered in the Mererani Hills of Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. It is used as a gemstone...

 over the years) or can be quite stable (such as those of diamonds). In general per carat prices of larger stones are higher than those of smaller stones, but popularity of certain sizes of stone can affect prices. Typically prices can range from 1USD/carat for a normal amethyst to US$20,000–50,000 for a collector's three carat pigeon-blood almost "perfect" ruby.

Grading

In the last two decades there has been a proliferation of certification for gemstones.
There are a number of laboratories which grade and provide reports on diamonds.
  • International Gemological Institute (IGI
    IGI
    IGI may refer to:* Le Igi is a traditional Samoan guitar tuning style* Indira Gandhi International Airport, Delhi* Interconnector Greece – Italy, a planned natural gas pipeline* International Genealogical Index* Indian Gemmological Institute...

    ), independent laboratory for grading and evaluation of diamonds, jewellery and colored stones.
  • Gemological Institute of America
    Gemological Institute of America
    The Gemological Institute of America, or GIA, is a nonprofit institute dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect all buyers and sellers of gemstones by setting and maintaining the standards used to evaluate...

     (GIA), the main provider of education services and diamond grading reports
  • Hoge Raad voor Diamant (HRD Antwerp), The Diamond High Council, Belgium is one of Europe's oldest laboratories. Its main stakeholder is the Antwerp World Diamond Centre.
  • American Gemological Society (AGS) is not as widely recognized nor as old as the GIA.
  • American Gem Trade Laboratory which is part of the American Gem Trade Association (AGTA), a trade organization of jewelers and dealers of colored stones.
  • American Gemological Laboratories (AGL), which was sold by "Collector's Universe" a NASDAQ
    NASDAQ
    The NASDAQ Stock Market, also known as the NASDAQ, is an American stock exchange. "NASDAQ" originally stood for "National Association of Securities Dealers Automated Quotations". It is the second-largest stock exchange by market capitalization in the world, after the New York Stock Exchange. As of...

     listed company which specializes in certification of collectibles such as coins and stamps. It is now owned by Christopher P. Smith, who was awarded the Antonio C. Bonanno Award for Excellence in Gemology in 2009
  • European Gemological Laboratory (EGL), founded in 1974 by Guy Margel in Belgium.
  • Gemmological Association of All Japan (GAAJ-ZENHOKYO), Zenhokyo, Japan, active in gemological research
  • Gemmological Institute of Thailand (GIT) is closely related to Chulalongkorn University
  • Gemmology Institute of Southern Africa, Africa's premium gem laboratory.
  • Asian Institute of Gemmological Sciences (AIGS), the oldest gemological institute in South East Asia, involved in gemological education and gem testing
  • Swiss Gemmological Institute (SSEF), founded by Prof. Henry Hänni, focusing on colored gemstones and the identification of natural pearls
  • Gübelin Gem Lab, the traditional Swiss lab founded by Dr. Eduard Gübelin. Their reports are widely considered as the ultimate judgement on high-end pearls, colored gemstones and diamonds.


Each laboratory has its own methodology to evaluate gemstones. Consequently a stone can be called "pink" by one lab while another lab calls it "Padparadscha". One lab can conclude a stone is untreated, while another lab concludes that it is heat treated. To minimise such differences, seven of the most respected labs, i.e. AGTA-GTL (New York), CISGEM (Milano), GAAJ-ZENHOKYO (Tokyo), GIA (Carlsbad), GIT (Bangkok), Gübelin (Lucerne) and SSEF (Basel), have established the Laboratory Manual Harmonisation Committee (LMHC), aiming at the standardization of wording on reports and certain analytical methods and interpretation of results. Country of origin has sometimes been difficult to find agreement on due to the constant discovery of new locations. Moreover determining a "country of origin" is much more difficult than determining other aspects of a gem (such as cut, clarity etc.).

Gem dealers are aware of the differences between gem laboratories and will make use of the discrepancies to obtain the best possible certificate.

Cutting and polishing

A few gemstones are used as gems in the crystal or other form in which they are found. Most however, are cut and polished for usage as jewelry. The picture to the left is of a rural, commercial cutting operation in Thailand. This small factory cuts thousands of carats of sapphire annually. The two main classifications are stones cut as smooth, dome shaped stones called cabochon
Cabochon
A cabochon , from the Middle French caboche , is a gemstone which has been shaped and polished as opposed to faceted. The resulting form is usually a convex top with a flat bottom. Cutting en cabochon is usually applied to opaque gems, while faceting is usually applied to transparent stones...

s, and stones which are cut with a faceting machine
Faceting machine
A faceting machine is broadly defined as any device that allows the user to place and polish facets onto a mineral specimen. Machines can range in sophistication from primitive jamb-peg machines to highly refined, and highly expensive, commercially available machines. A major division among...

 by polishing small flat windows called 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 at regular intervals at exact angles.

Stones which are opaque such as 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...

, turquoise
Turquoise
Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl648·4. It is rare and valuable in finer grades and has been prized as a gem and ornamental stone for thousands of years owing to its unique hue...

, variscite
Variscite
Variscite is a hydrated aluminium phosphate mineral |H2O]]). It is a relatively rare phosphate mineral. It is sometimes confused with turquoise; however, variscite is usually greener in color....

, etc. are commonly cut as cabochons. These gems are designed to show the stone's color or surface properties as in opal and star sapphires. Grinding wheels and polishing agents are used to grind, shape and polish the smooth dome shape of the stones.

Gems which are transparent are normally faceted, a method which shows the optical properties of the stone's interior to its best advantage by maximizing reflected light which is perceived by the viewer as sparkle. There are many commonly used shapes for faceted stones
Diamond cut
A diamond cut is a style or design guide used when shaping a diamond for polishing such as the brilliant cut. Cut does not refer to shape , but the symmetry, proportioning and polish of a diamond...

. The facets must be cut at the proper angles, which varies depending on the optical properties of the gem. If the angles are too steep or too shallow, the light will pass through and not be reflected back toward the viewer. The faceting machine is used to hold the stone onto a flat lap for cutting and polishing the flat facets. Rarely, some cutters use special curved laps to cut and polish curved facets.

Color

The color of any material is due to the nature of light itself. Daylight, often called white light, is actually a mixture of different colors of light. When light passes through a material, some of the light may be absorbed, while the rest passes through. The part that is not absorbed reaches the eye as white light minus the absorbed colors. A ruby appears red because it absorbs all the other colors of white light (blue, yellow, green, etc.) except red.

The same material can exhibit different colors. For example ruby and sapphire have the same chemical composition (both are 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...

) but exhibit different colors. Even the same gemstone can occur in many different colors: sapphires show different shades of blue and pink and "fancy sapphires" exhibit a whole range of other colors from yellow to orange-pink, the latter called "Padparadscha sapphire".

This difference in color is based on the atomic structure of the stone. Although the different stones formally have the same chemical composition, they are not exactly the same. Every now and then an atom is replaced by a completely different atom (and this could be as few as one in a million atoms). These so called impurities are sufficient to absorb certain colors and leave the other colors unaffected.

For example, beryl, which is colorless in its pure mineral form, becomes emerald with chromium impurities. If you add manganese instead of chromium, beryl becomes pink morganite. With iron, it becomes aquamarine.

Some gemstone treatments make use of the fact that these impurities can be "manipulated", thus changing the color of the gem.

Treatment

Gemstones are often treated to enhance the color or clarity of the stone. Depending on the type and extent of treatment, they can affect the value of the stone. Some treatments are used widely because the resulting gem is stable, while others are not accepted most commonly because the gem color is unstable and may revert to the original tone.

Heat

Heat can improve gemstone color or clarity. The heating process has been well known to gem miners and cutters for centuries, and in many stone types heating is a common practice. Most citrine is made by heating amethyst, and partial heating with a strong gradient results in ametrine
Ametrine
Ametrine, also known as trystine or by its trade name as bolivianite, is a naturally occurring variety of quartz. It is a mixture of amethyst and citrine with zones of purple and yellow or orange...

—a stone partly amethyst and partly citrine. Much aquamarine is heated to remove yellow tones and change the green color into the more desirable blue or enhance its existing blue color to a purer blue.

Nearly all tanzanite is heated at low temperatures to remove brown undertones and give a more desirable blue/purple color. A considerable portion of all sapphire and ruby is treated with a variety of heat treatments to improve both color and clarity.

When jewelry containing diamonds is heated (for repairs) the diamond should be protected with boracic acid; otherwise the diamond (which is pure carbon) could be burned on the surface or even burned completely up. When jewelry containing sapphires or rubies is heated, it should not be coated with boracic acid or any other substance, as this can etch the surface; they do not have to be "protected" like a diamond.

Radiation

Virtually all blue topaz, both the lighter and the darker blue shades such as "London" blue, has been irradiated
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...

 to change the color from white to blue. Most greened quartz (Oro Verde) is also irradiated to achieve the yellow-green color.

Waxing/oiling

Emeralds containing natural fissures are sometimes filled with wax or oil to disguise them. This wax or oil is also colored to make the emerald appear of better color as well as clarity. Turquoise is also commonly treated in a similar manner.

Fracture filling

Fracture filling has been in use with different gemstones such as diamonds, emeralds and sapphires. In 2006 "glass filled rubies" received publicity. Rubies over 10 carat (2 g) with large fractures were filled with lead glass, thus dramatically improving the appearance (of larger rubies in particular). Such treatments are fairly easy to detect.

Synthetic and artificial gemstones

Some gemstones are manufactured to imitate other gemstones. For example, 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...

 is a synthetic diamond simulant
Diamond simulant
The high price of gem-grade diamonds, as well as significant ethical concerns of the diamond trade, have created a large demand for materials with similar gemological characteristics, known as diamond simulants or imitations. Simulants are distinct from synthetic diamond, which unlike simulants is...

 composed of 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...

 oxide. 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 :...

, also a synthetic stone, is another example. The imitations copy the look and color of the real stone but possess neither their chemical nor physical characteristics. Moissanite actually has a higher refractive index than diamond and when presented beside an equivalently sized and cut diamond will have more "fire" than the diamond.

However, lab created gemstones are not imitations. For example, diamonds, ruby
Ruby
A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum . The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires...

, 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...

s and 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...

s have been manufactured in labs to possess identical chemical and physical characteristics to the naturally occurring variety. Synthetic (lab created) 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...

s, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller 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...

s have been manufactured in large quantities as industrial abrasive
Abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...

s, although larger gem-quality synthetic diamonds are becoming available in multiple carats.

Whether a gemstone is a natural stone or a lab-created (synthetic) stone, the characteristics of each are the same. Lab-created stones tend to have a more vivid color to them, as impurities are not present in a lab and do not modify the clarity or color of the stone.

External links

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