Adularescence
Adularescence(/ˌædʒələˈrɛsəns/AJ-ə-lə-RES-əns) is anoptical phenomenonthat is produced ingemstoneslikemoonstone.The optical effect is similar tolabradorescenceandaventurescence.
Description
[edit]The effect of adularescence, also commonly referred to asschillerorshiller,is best described as a milky, bluishlusteror 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).[1]Though white schiller is the most common, in rarer specimens, orange or blue lusters are produced.[2]
This effect is most typically produced byadularia,aK-feldsparororthoclase(KAlSi3O8), from which the name is derived.[3][4][5]Adularescence appears in numerous other gemstones, notably commonopal,rose quartzandagate.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 – the "girasol effect" andopalescence(for opals only) are two such terms. When the schiller forms an indistinct band, it is said to display achatoyant effect.Only clearly defined bands are referred to as "cat's eyes".[5]
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 at a scale near the wavelength of light (approximately 0.5micron) – this leads tolight scattering and interference.[6]
See also
[edit]References
[edit]- ^Long, Bill (2004), "Varieties of Light II: Pleochroism, Nacreous, Adularescence, A(d)venturescenceArchived2008-12-29 at theWayback Machine."
- ^Semi Precious Stones Guide: List of Gemstone Phenomena
- ^King, R. J. (1990). "Minerals explained 12: Alkali feldspar (Part 2)."Geology Today,6 pp. 27-29.
- ^"Adularescence,"reference.com
- ^abShipley, Robert M. (2007),Dictionary of Gems and Gemology,Read Books, p. 93.
- ^Nassau, Kurt (1978). "The origins of color in minerals,"American Mineralogist,vol. 63, pp. 219-229.