Orthoclase is best known for moonstone. It is occasionally a transparent, faceted gem. Note that moonstone is occasionally a labradorite.
Highest values go to adularescence, transparency, size.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Is a Variety of||Feldspar|
|Varieties||Adularia, Moonstone, Sanidine|
|Colors||Colorless white gray black green orange yellow brown|
|Fracture Luster||Pearly to vitreous|
|Fracture||Uneven to splintery|
|Cleavage||Perfect and easy in two directions, parting common|
|Stone Sizes||To large|
|Enhancements||Dark coating on back, enhances adularescence, rare, detect with magnification|
|Transparency||Transparent to opaque|
|Phenomena||Adularescence, asterism, chatoyancy|
|Identifying Characteristics||Centipede inclusions Adularia|
|Formula||KAlSi3O8 (Note, this is the same formula as Microcline, but a different crystal system.)|
|Pleochroism||Usually none. Transparent yellow stones weak to moderate, two shades of yellow|
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 moonstone is also found and will be discussed with plagioclase moonstones.
Orthoclase is from Greek words meaning break straight because the cleavages are at 90°. Adularia is also a locality-derived name from Adular-Bergstock, Switzerland, where the variety occurs.
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).
- Moonstone: LW inert or blue. SW orange. May fluoresce weak pink to moderate red, LW and SW.
- Yellow orthoclase: LW and SW inert to weak reddish orange
- Semitranslucent: LW white or red
- Moonstone: Not diagnostic
- Yellow Orthoclase: Broad bands at 420 and 448 nm
Variety and Trade Names
- Adularia, any color with adularescence
- Moonstone, any color with adularescence
- Moonstone, is also used for nonphenominal orthoclase