Moonstone

moonstone
Moonstone cabochons

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

Cause of Color :

Color due to impurities, adularescence and sheen due to scattering or light.

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.

Crystal System / Forms :

Monoclinic System

Cuts & Uses :

Facetted, cabochon, beads and carving.

Dispersion : 0.012
Hardness : 6
Lustre : Vitreous
Magnification :

Fine stress cracks parallel to cleavage directions known as centipede inclusions, fingerprints and crystal inclusions.

Optic Character :

Anisotropic, D.R.; Biaxial negative

Pleochroism : Not common
Refractive Index / Birefringence :

1.518 – 1.526 / 0.008

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)
Sources :

Sri Lanka, Myanmar, India, Madagascar, U.S.A., Tanzania.

Spectrum :

Not characteristic.

Synthesis :

None.

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