Orthoclase Value, Price, and Jewelry Information

Rough orthoclase

Orthoclase

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

Orthoclase Value

Highest values go to adularescence, transparency, size.

The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.

Moonstone Value via Gem Price Guide

Rainbow Moonstone (Peristerite) .5 to .99 carats 1 to 7 carats
Cabochons to /ct to /ct
Labradorite 10 to 30 carats
Cabochons to /ct
White, including cat's eyes All sizes
to /ct
White – Adularescent All sizes
to /ct
Other Colors All sizes
to /ct

Orthoclase Information

Data Value
Name Orthoclase
Is a Variety of Feldspar
Varieties Adularia, Moonstone, Sanidine
Colors Colorless, white, gray, black, green, orange, yellow, brown
Crystallography Monoclinic.
Refractive Index 1.518-1.539
Luster Vitreous, pearly.
Polish Luster Vitreous.
Fracture Luster Vitreous, pearly.
Hardness 6-6.5
Wearability Poor
Fracture Uneven to splintery. Parting common
Specific Gravity 2.55-2.63
Birefringence 0.004-0.010
Cleavage Perfect and easy in two directions
Dispersion 0.012
Stone Sizes To large
Heat Sensitivity Yes
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
Enhancements Dark coating on back, enhances adularescence, rare, detect with magnification
Typical Treatments Surface Coating
Transparency Transparent to opaque
Absorption Spectrum Strong line at 4200. Bands at 4450 and 4200. Yellow Orthoclase: broad bands at 4200 and 4480. Moonstone: not diagnostic.
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

Optics

Biaxial (-). 2V = 33-103°.

Optic Sign Biaxial -

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. Orthoclase moonstone is also found and will be discussed with plagioclase moonstones.

Name

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.

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).

UV

  • 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

Absorption Spectrum

  • 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