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Moonstone (Feldspar)

Chemistry:
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  More Information:

KAlSi₃O₈
Silicates: Tectosilicates
8/J.06-40 (Feldspar group)
(Buddingtonite - Slawsonite series)
Monoclinic - Prismatic
Vitreous to pearly
6.0 - 6.5
2.56 - 2.59
0.012
1.520 - 1.525  Biaxial ( - )
0.005
Mindat.org (Moonstone)
Mindat.org (Orthoclase)
Webmineral.com (Orthoclase)
 

Moonstone is a member of the Feldspar Group of minerals that also includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. Moonstone is a popular name for Orthoclase and/or Plagioclase Feldspar showing a blue sheen. The blue sheen is produced by the presence of very finely dispersed Albite crystals or plates within a body of Orthoclase or Labradorite. This blue sheen is refered to as adularescence. Blue sheen Moonstone with a colorless and transparent body free of inclusions makes for a rare and beautiful gem. Common Moonstone is usually translucent to opaque or heavily included.

Moonstone gets its name from its blueish glow due to adularescence. The name Orthoclase is from the Greek words orthos meaning right and kalo meaning I cleave because its cleavages are at right angles. Feldspar is from the Swedish feldt + spat meaning that it was found in fields overlying granite.
 

We have not photographed our Plagioclase Moonstone gems yet.  Please check back soon!