Rough orthoclase
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.
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| 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 |
| 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.
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
Absorption Spectrum
Variety and Trade Names
Gemstone luminescence is a glow that occurs when certain stones are subjected to energy such as UV light. This effect can help gemologists identify…
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