Category  :  Silicate mineral
Hardness  :  6
Gravity  :  2.55 - 2.63
Reflection  :  1.53 / 1.52
Color  :  Colorless in plane-polarized light, commonly with a dusty or cloudy appearance; white to pink in hand-sample
Chemical formula  :  KAlSi3O8
Crystal system  :  Monoclinic
Major varieties  :  Noble Orthoclase, Adularia Moonstone

The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. An alternate name is alkali feldspar. The gem known as moonstone (see below) is largely composed of orthoclase.

Together with the other potassium feldspars orthoclase is a common raw material for the manufacture of some glasses, some ceramics, such as porcelain, and as a constituent of scouring powder.

Some intergrowths of orthoclase and albite have an attractive pale lustre and are called moonstone when used in jewelry. Most moonstones are translucent and white, although grey and peach-colored varieties also occur. In gemology, their lustre is called adularescence and is typically described as creamy or silvery white with a "billowy" quality. It is the state gem of Florida.

The gemstone commonly called rainbow moonstone is more properly a colorless form of labradorite and can be distinguished from "true" moonstone by its greater transparency and play of color, although their value and durability do not greatly differ.

Noble Orthoclase

The color varies from mid-to golden yellow. It is perfectly transparent with vitreous luster. The cut most commonly used with this gem is the step cut. Gems are generally free of inclusions. It resembles softly colored citrine and beryl. It is neither imitated nor produced synthetically.

Adularia Moonstone

Because of its slightly turbid transparency, gems cut en cabochon show a mobile reflection, which is softer and more diffuse than that of chatoyant stones. Moonstone generally has an almost transparent background, which is practically colorless, pale gray, or tinged with yellow, with a whitish to silvery white or blue shimmer. Incipient cleavage cracks may be visible inside the stone. The type with a blue reflection is highly prized.

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