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While feldspar is one of the most abundant mineral families in the world, gem quality crystals are scarce, coveted and spectacularly beautiful, as they often possess rare optical effects. Many feldspar gems only occur in isolated deposits and are far rarer than better known gems such as diamond, ruby or sapphire. Some varieties are commonly encountered and many are seen in granites, for example, "Black Pearl" granite countertops are actually composed mostly of feldspar. While orthoclase, a potassium-rich feldspar, is better known to collectors, there is one variety that many are familiar with moonstone, prized for its shimmering adularescence.

Orthoclase Polished
Orthoclase Classification
Common Name Orthoclase
Species Feldspar
Orthoclase Optical Properties
Transparency Transparent - Opaque
Dispersion Strength: Weak Fire Value: 0.120
Refractive Index 1.518-1.526 Tolerance: (+0.010)
Birefringence 0.005-0.008
Optic Character Biaxial
Optic Sign Negative
Polariscope Reaction Aggregate (AGG), Doubly Refractive (DR)
Fluorescence SWUV: transparent yellow: inert to weak reddish orange
LWUV: transparent yellow: inert to weak reddish orangesemitranslucent white: red
Pleochroism Unobservable
Orthoclase Characteristic Physical Properties
Hardness 6-6.5
Streak White
Specific Gravity 2.550-2.610 Range:+/-0.03 Typical:2.580
Toughness Poor
Inclusions Orthoclase feldspar might display centipede like inclusions. Material with aventuresence or asterism might contain reflective inclusions such as fine needles or elongated hematite platelets.
Luster Vitreous
Fracture Uneven, Splintery
Cleavage Perfect, in two directions
Orthoclase Chemistry & Crystallography
Chemical Name potassium aluminum silicate
Chemical Formula KAlSi3O8
Crystal System Monoclinic
Chemistry Classification Silicate

Orthoclase Colors

  • Yellow Orthoclase Yellow
  • Yellow Orthoclase Yellow
  • Green Orthoclase Green
  • Colorless Orthoclase Colorless
  • White Orthoclase White
  • Pink Orthoclase Pink
  • Green Orthoclase Green
  • Orange Orthoclase Orange
  • Brown Orthoclase Brown
  • Gray Orthoclase Gray
  • Yellow Orthoclase Yellow
  • Brown Orthoclase Brown
  • Black Orthoclase Black
  • Orange Orthoclase Orange
  • Brown Orthoclase Brown

Alternate Names

Adularia, Moonstone, Sunstone

Countries of Origin

Pakistan; United States of America (the); Brazil; Italy; Madagascar; India

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Tim Matthews


Tim Matthews

Tim Matthews is President and Chief Executive Officer of Jewelry Television® (JTV), as well as a member of the company's Board of Directors. He oversees and leads all aspects of the company's powerful omni-digital retail platform that uniquely specializes in fine jewelry and gemstones. His passion for business and gemstones has led him to become a recognized expert in the field of gemology. He is a life member of the Gemmological Association of Great Britain (Gem-A) and has earned Gem-A's highest degrees, the Gemmology (FGA) and Diamond (DGA) diplomas. He is also a Graduate Gemologist (GG) of the Gemological Institute of America (GIA) and has also completed GIA's Graduate Diplomas in Diamonds, Colored Stones and Pearls. Under his leadership, JTV has become the leader in the sourcing and selling of color gemstones and jewelry.

This page was created on June 27, 2014.

This page was last edited on October 24, 2019.