What is Adularescence?

What is Adularescence?
Moonstone’s unearthly glow is caused by light scattering between microscopic layers of feldspar.
Photo: G. Cedric

Adularescence is an optical phenomenon that is produced in gemstones such as moonstones.  Adularescence is the metallic iridescence originating from below the surface of a stone, that occurs when light is reflected between layers of minerals.

The effect of adularescence, also commonly referred to as schiller or shiller, 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).

This effect is most typically produced by adularia, 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.

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 (c. 0.5 micron) – this leads to light scattering and interference.

Cause of Adularescence in Blue Moonstone

Blue Moonstone is a gem from the feldspar group and is composed of layers of albite – potassium rich aluminium silicate and orthoclase – sodium rich aluminum silicate. The schiller is produced because of light interference caused by the light having to weave its way through the layers with slightly different optical properties. The adularescence is caused by scattered light passing through the exsolution lamellae that act as scattering centres, creating a bluish hue/ lustre.

See also: 
How Do Asterism Minerals Form?
How to Identify Minerals in 10 Steps (photos)
How Do Agates Form?
The Only Diamond Mine In the World Where You Can Be the Miner
Top Spots For Gem Hunting In The US
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