MOONSTONE A ghostly sheen moves under the surface of this feldspar, like moonlight glowing in water.
Moonstone is a variety of the feldspar-group mineral orthoclase. During formation, orthoclase and albite separate into alternating layers. When light falls between these thin layers it is scattered producing the phenomenon called adularescence. Adularescence is the light that appears to billow across a gem. Other feldspar minerals can also show adularescence including labradorite and sanidine.
Moonstone’s unearthly glow is caused by light scattering between microscopic layers of feldspar.
The minerals in the feldspar family make up more than half of the Earth’s rocky crust.
Feldspar layers that create moonstone’s sheen are similar to the size of a wavelength of light.
- Mineral: Feldspar
- Chemistry: KAlSi3O8
- Color: Colorless to White, Gray, Green, Peach, Brown
- Refractive index: 1.518 to 1.526
- Birefringence: 0.05 to 0.008
- Specific gravity: 2.58
- Mohs Hardness: 6.0 to 6.5
An assessment of the following characteristics determines moonstone’s value.
The finest moonstone is a gem of glassy purity with a mobile, electric blue shimmer.
Characteristic inclusions include tiny tension cracks called centipedes.
As it displays moonstone’s phenomena to best advantage, cabochon is the common cut.
Moonstone comes in a wide range of sizes and carat weights.