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Labradorite is a gem variety of plagioclase feldspar. Plagioclase itself being an isomorphous mixture of albite (sodium aluminium silicate) and anorthite (calcium aluminium silicate). It is usually polished as a cabochon. Its beauty as a gemstone is mainly because of its schiller which is caused by interference effects of light due to fine lamellae (layers or plates) formed by repeated twinning of its crystals, and also by the inclusion of tiny platelets of impurities such as magnetite, an iron oxide.

Collins English Dictionary gives schiller as

an unusual metallic lustre in some minerals caused by internal reflection from certain inclusions such as gas cavities. (C19 from German schiller irridescence, from Old High German scilihen to blink.
A similar effect can also be seen when tiny snow or ice crystals are carried in the air in strong sunlight, and the sunlight is seen to reflect strongly in a rainbow-like display of blinking multi-coloured prismatic light. Alpine mountain men also call this schiller. Seeing it is one of the transient highs to be found on a ski holiday. It is best viewed off-piste with a can of cold beer after a descent of a remote couloir.

The body colour of labradorite is grey, and the schiller is usually blue and green, but yellow, gold, red and purple are among the other irridescent colours displayed. The flashes of colour in labradorite have been compared to the wings of tropical butterflies.

Chemical Composition and NameNaAlSi3O8 - Sodium Aluminium Silicate
& CaAl2Si2O8 - Calcium Aluminium Silicate
Hardness6 - 6.5
Specific Gravity2.69
Crystalline SystemMonoclinic
Refractive Index1.560 - 1.568
Bi-refringence0.001 or 0.008
Optic SignPositive
Optical CharacterBiaxial the Lowest Possible Price

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