Orthoclase

Specific Gravity: 2.56
Refractive Index: 1.51-1.54
Birefringence (DR): 0.005
Luster: Vitreous
Crystal Structure: Monoclinic
Composition: Potassium aluminum silicate
Hardness: 6

Orthoclase, an alkali feldspar, occurs in a range of colors. The most common is colorless. Adularia is a transparent variety from Adular-Bergstock in Switzerland. It has a bluish-white ‘schiller’ or sheen, called adularescence. Some adularia crystals may show internal cracks or flaws.

The yellow orthoclase is usually faceted as a step-cut because the stone is often fragile. The yellow color is due to the presence of iron. An Orthoclase crystal is usually in the form of a columnar or tabular prism and is often twinned with another crystal.

Feldspars are the most common rock-forming minerals at the Earth’s surface. They are divided into two groups: the alkali feldspar and the plagioclase feldspar. Orthoclase gets its name from the Greek for ‘break straight’, a reference to the stone’s perfect cleavage at near 90 degrees.

Orthoclase forms in igneous and metamorphic rocks. Which type is formed depends on the temperature it forms at, and how it cools. It is common to find orthoclase crystals in a translucent to semi-translucent form.

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Occurrence

Orthoclase occurs in intrusive igneous rocks and is one of the main constituents of granitic pegmatites. It is also found in metamorphic rocks such as gneiss and schist. Clear, colorless orthoclase occurs in Madagascar. Yellow and colorless cuttable material, cat’s eye, and some star stones occur in the gem gravels of Myanmar (Burma) and Sri Lanka.

The best yellow orthoclase occurs in Madagascar in pegmatites and may be faceted for collectors. Yellow orthoclase from Germany and Madagascar may be cut en cabochon to show the cat’s-eye effect.