Orthoclase Value, Price, and Jewelry Information

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Rough orthoclase

Orthoclase is best known for moonstone. It is occasionally a transparent, faceted gem.

Orthoclase Value

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Highest values go to adularescence, transparency, size.

Orthoclase Information

Data	Value
Name	Orthoclase
Is a Variety of	Feldspar
Varieties	Adularia, Moonstone, Sanidine
Stone Sizes	To large
Heat Sensitivity	Yes
Identifying Characteristics	Centipede inclusions Adularia
Formula	KAlSi3O8 (Note, this is the same formula as Microcline, but a different crystal system.)
Colors	Colorless, white, gray, black, green, orange, yellow, brown
Fracture	Uneven to splintery. Parting common
Hardness	6-6.5
Cleavage	Perfect and easy in two directions
Wearability	Poor
Crystallography	Monoclinic.
Crystallographic Forms
Refractive Index	1.518-1.539
Birefringence	0.004-0.010
Dispersion	0.012
Luminescence	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. White to violet in X-rays.
Luminescence Present	Yes
Luminescence Type	Fluorescent, UV-Long, UV-Short, X-ray Colors
Absorption Spectrum	Strong line at 4200. Bands at 4450 and 4200. Yellow Orthoclase: broad bands at 4200 and 4480. Moonstone: not diagnostic.
Pleochroism	Usually none. Transparent yellow stones weak to moderate, two shades of yellow
Optics	Biaxial (-). 2V = 33-103°.
Optic Sign	Biaxial -
Luster	Vitreous, pearly.
Polish Luster	Vitreous.
Fracture Luster	Vitreous, pearly.
Specific Gravity	2.55-2.63
Enhancements	Dark coating on back, enhances adularescence, rare, detect with magnification
Typical Treatments	Surface Coating
Transparency	Transparent to opaque
Phenomena	Adularescence, asterism, chatoyancy

Comments

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.

Name

Orthoclase is from Greek words meaning break straight because the cleavages are at 90°.

Occurrence

A component of many rocks, especially alkalic and plutonic acid rocks, also granites, pegmatites, syenites.

Many localities in the United States.
Canada.
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.

Stones Sizes

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

Absorption Spectrum