Adularescence
Encyclopedia
Adularescence is an optical phenomenon
, similar to labradorescence
and aventurescence
, produced most notably by moonstones
. Adularescence is also commonly referred to as schiller or shiller. The effect is best described as a milky, bluish luster
or glow originating from below the surface of the gemstone. The schiller, appearing to move as the stone is turned (or as the light source is moved), gives the impression of lunar light floating on water (accounting for moonstone's name). Though white schiller is the most common, in rarer specimens, orange or blue lusters are produced.
This effect is most typically produced by adularia (also known as precious moonstone), from which the name derives. Adularescence appears in numerous other gemstones, notably common opal
, rose quartz and agate
. However, due to inclusions in these other stones, the effect is displayed differently. The schiller is scattered by inclusions and appears hazy; non-hazy specimens are specially referred to as "milky". Thus, adularescence occurring in non-adularia gemstones is termed differently - "girasol effect" and opalescence (for opals only) are two such terms. When the schiller forms an indistinct band it is said to display a chatoyant effect
. Only clearly defined bands are referred to as "cat's eyes".
As an optical phenomenon, adularescence exists only in the presence of light; it is a product of the interaction between light and the internal microstructures of the mineral and not a property of the mineral itself. The effect is produced by alternating layers of two types (in moonstone feldspar
the layers are orthoclase
and albite
) at a scale near the wavelength of light (ca. 0.5 micrometer) - this leads to light scattering.
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
, similar to labradorescence
Labradorite
Labradorite , a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573....
and aventurescence
Aventurescence
In gemology, aventurescence is an optical reflectance effect seen in certain gems. The effect amounts to a metallic glitter, arising from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they also influence the material's body colour...
, produced most notably by moonstones
Moonstone (gemstone)
Moonstone is a sodium potassium aluminium silicate, with the chemical formula AlSi3O8.-Etymology:Its name is derived from a visual effect, or sheen, caused by light reflecting internally in the moonstone from layer inclusion of different feldspars....
. Adularescence is also commonly referred to as schiller or shiller. The effect is best described as a milky, bluish luster
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 glow originating from below the surface of the gemstone. The schiller, appearing to move as the stone is turned (or as the light source is moved), gives the impression of lunar light floating on water (accounting for moonstone's name). Though white schiller is the most common, in rarer specimens, orange or blue lusters are produced.
This effect is most typically produced by adularia (also known as precious moonstone), from which the name derives. Adularescence appears in numerous other gemstones, notably common 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...
, rose quartz and agate
Agate
Agate is a microcrystalline variety of silica, chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks and can be common in certain metamorphic rocks.-Etymology...
. However, due to inclusions in these other stones, the effect is displayed differently. The schiller is scattered by inclusions and appears hazy; non-hazy specimens are specially referred to as "milky". Thus, adularescence occurring in non-adularia gemstones is termed differently - "girasol effect" and opalescence (for opals only) are two such terms. When the schiller forms an indistinct band it is said to display a chatoyant effect
Chatoyancy
In gemology, chatoyancy , or chatoyance, is an optical reflectance effect seen in certain gemstones. Coined from the French "œil de chat," meaning "cat's eye," chatoyancy arises either from the fibrous structure of a material, as in tiger eye quartz, or from fibrous inclusions or cavities within...
. Only clearly defined bands are referred to as "cat's eyes".
As an optical phenomenon, adularescence exists only in the presence of light; it is a product of the interaction between light and the internal microstructures of the mineral and not a property of the mineral itself. The effect is produced by alternating layers of two types (in moonstone feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
the layers are orthoclase
Orthoclase
Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...
and albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
) at a scale near the wavelength of light (ca. 0.5 micrometer) - this leads to light scattering.