Orthoclase is a mineral with a hardness of 6 out of 10 on the Mohs scale of mineral hardness [?]. These Monoclinicly structured gems are made of potassium aluminum silicate, their full chemical compound being Kalsi3O8.
Orthoclase mineral is a silicate of potassium and aluminum, belonging to the Feldspar group. It has the same composition as microcline, but is stable at slightly higher temperatures.
It occurs as prismatic, sometimes flat-sided crystals, but in rocks it is usually anhedral. It may be perfectly transparent and yellow or almost colorless, but it is more often semi-opaque and white to grayish-white, yellowish white, pink, or reddish.
Clear, yellow orthoclase from the pegmatites of the Malagasy Republic is well-known and is used as a gemstone. Fine pink crystals are found in cavities in granite at Baveno (Novara) and in granophyre at Cussao al Monte (Varese), and white crystals at San Piero in Campo (Elba), all in Italy.
Classic twinned forms come from Carlsbad (Czechoslovakia), and well-formed red crystals from Kirkpatrick (Scotland), Flims (Grison, Switzerland), Marienburg (Germany), Striegau (Silesia), and various localities in the Soviet Republics. Blue crystals occur near Lake Baikal (Russia), gray at Villadreu (Spain). Remarkable crystals are found in lithoclases of crystalline-schistose rocks in the Zilleral (Austria), and Val di Vizze (Bolzano, Italy).
The somewhat rare variety of transparent orthoclase from Malagasy is called noble orthoclase. It is basically yellow in color and usable as a gem.
Its color varies from mid- to golden yellow, champagne-colored. It is perfectly transparent with vitreous luster. Gems are usually free of inclusions.
Noble orthoclase can be confused with several yellow stones.
Orthoclase got its name because it has two directions of cleavage, which are at right angles to one another.
Orthoclase mineral is important for industrial uses. The mixture orthoclase, kaolin, and quartz is easy to mold and fuses at a relatively low temperature, to a light, glassy, white, slightly porous mass known as porcelain. When very pure, it is used to make special porcelains (high-tension electric insulators, ceramic glazes, and dental products) and opalescent glass. For some years it has been used, finely ground and mixed with detergents, as a scouring powder.
The cut most commonly used with gem-quality noble orthoclase is the step cut. Its value is low, even compared with other secondary gems. But being somewhat rare, fine specimens are sought by collectors and connoisseurs. Noble orthoclase is neither imitated nor produced synthetically.