Moonstone

Moonstone is the opalescent variety of orthoclase, with a blue or white sheen (or "schiller"), rather like the shine of the moon after which it is named. This is caused by the reflection of light from the internal structure, made up of alternating layers of albite and orthoclase feldspar. Thin albite layers give an attractive blue; thicker layers produce a white "schiller". Stones of large size and fine quality are rare.

The best material is from Burma and Sri Lanka. Other localities include India, Madagascar, Brazil, United States, Mexico, Tanzania and the European Alps.

Moonstone Pictures

Moonstone cabochons

Moonstone Properties

Chemical Composition: 
Aluminum silicate of potassium, sodium and calcium. KAlSi3O8. Isomorphous series made up of Albite (Na), Oligoclase, Andesine, Labradorite, Bytownite and Anorthite (Ca) where the two end member are NaAlSi3O8 and CaAl2Si2O8.
Colors / Varieties: 
Orthoclase: Moonstone (all colors); Transparent: yellow, pink and white; Phenomena: sheen, chatoyancy, asterism, adularescence (blue sheen)
Crystal System / Forms: 
Monoclinic System
Hardness: 
6
Specific Gravity: 
2.55 - 2.63
Cleavage / Fracture: 
Easy 2 directional / Uneven.
Optic Character: 
Anisotropic, D.R.; Biaxial negative
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.518 - 1.526 / 0.008
Pleochroism: 
Not common
Dispersion: 
0.012
Magnification: 
Fine stress cracks parallel to cleavage directions known as centipede inclusions, fingerprints and crystal inclusions.
U.V. Fluorescence: 
Pink under shortwave commonly, variable.
Spectrum: 
Not characteristic.
Cause of Color: 
Color due to impurities, adularescence and sheen due to scattering or light.
Treatment (Enhancement): 
None.
Synthesis: 
None.
Simulants (with separation tests): 
  • Moonstone from chalcedony (structure, phenomena); glass (inclusions); opal (R.I., S.G.)
  • Transparent orthoclase from quartz (optic figure); scapolite (optic figure); petalite (S.G., U.V. fluorescence), glass (optic character, inclusions, U.V. fluorescence)
Geological Occurrence: 
Igneous rocks, pegmatite.
Sources: 
Sri Lanka, Myanmar, India, Madagascar, U.S.A., Tanzania.
Cuts & Uses: 
Facetted, cabochon, beads and carving.