Victorian Cat's-Eye Orthoclase Feldspar and Diamond Earrings. Photo Courtesy of Lang Antiques.

Feldspar is a gem group that includes microcline, orthoclase and plagioclase varieties. Each variety varies as to crystal structure, refractive index, color and other specifics. Gemstones include: amazonite, moonstone, labradorite, sunstone. From the Greek word klasis meaning cleaving (clase) with a cleavage direction as the prefix.

• Microcline
  • Amazonite
• Orthoclase
  • Moonstone
• Plagioclase
  • Labradorite
  • Oligoclase
    • Spectrolite (trade name for Labradorite)
    • Sunstone (variety of Labradorite or Oligoclase depending on the color)

Gemological information

Gemological Information for Feldspar
Color	Light Green, Greenish Blue, White for Microcline, Green, Colorless, White, Yellow, Brown, Gray and Black for Orthoclase, Gray to Black for Labradorite, Orange, Brown, Colorless for Oligoclase
Crystal Structure	Triclinic (microcline) Monoclinic (orthoclase and plagioclase)
Refractive Index	1.522 to 1.530 (microcline) 1.518 to1.526(orthoclase) 1.599 to 1.568 Labradorite, 1.537 to 1.547 Oligoclase
Durability	Poor
Hardness	6 to 6.5
Similar Stones	Aventurine Quartz, Chalcedony, Turquoise
Treatments	Wax Impregnation, Irradiation
Country of Origin	Worldwide Deposits
Feldspar Care
Ultrasonic Cleaning	Never
Steam Cleaning	Never
Warm Soapy Water	Safe
Chemical Attack	Hydrofluoric Acid attacks quickly. Impurities may react to other acids
Light Sensitivity	Stable
Heat Sensitivity	Can Crack, Fade and Cleave

Sources

• Gem Reference Guide, Los Angeles: Gemological Institute of America, 1988. Pp. 101-107.

• Newman, Harold. An Illustrated Dictionary of Jewelry, New York: Thames and Hudson, 1981. Pp. 122.

• Schumann, Walter. Gemstones of the World, New York: Sterling Publishing, Co., 1977. Pp. 164-167.