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Highest values go to adularescence, transparency, size.
Yellow and colorless catseye gems are known from Burma and Sri Lanka. Some of these (Sri Lankan) stones are also asteriated. Yellow faceted orthoclase is a handsome gemstone. Unfortunately, the cleavage makes it less advantageous for wear. Also, fine rough is hard to find, but large stones are displayed in museums.
Orthoclase is from Greek words meaning break straight because the cleavages are at 90°.
A component of many rocks, especially alkalic and plutonic acid rocks, also granites, pegmatites, syenites.
- Many localities in the United States.
- Switzerland: fine crystals, known as adularia (S.G. = 2.56), from the St. Gotthard Region: the material contains some Na.
- Itrongahy Madagascar: fine, transparent yellow orthoclase in large crystals, usually with rounded faces. (Indices 1.522/1.527; birefringence 0.005; S.G. 2.56.) Faceted gems may be very large and deep in color.
- Greenland: brownish transparent crystals to more than 2 inches.
- Tvedestrand, Norway: orthoclase sunstone, deep redorange, in masses up to a few inches in size.
- Sri Lanka: in the gem gravels.
- Burma: gravels.
Madagascar produces by far the largest cuttable orthoclase known.
Smithsonian Institution (Washington, D.C): 249.6 (yellow, Madagascar); 104.5 (pale green catseye, Sri Lanka); 22.7 (white star, Sri Lanka); 6.0 (colorless, North Carolina).
- Yellow orthoclase: LW and SW inert to weak reddish orange
- Semitranslucent: LW white or red
- Yellow Orthoclase: Broad bands at 420 and 448 nm
Variety and Trade Names