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A translucent, hydrated Silica mineral that is used as a Gemstone. Opal is amorphous, brittle and fractures conchoidally. It is lustrous with colors varying from pearly white, pale blue, gray, green, yellow, red and brown. High quality opals produce interference colors due to the presence of microscopic cracks that form when the stone hardens. The stones are often oiled to bring out the luster and deepen the color. Opals have been mined or gathered since the late iron age (500-50 BCE) as gemstones and ornamental stones. In Roman times, opals ranked second to emeralds as the most valuable gemstone. Fire opals have been mined in Mexico for over 500 years. Black and white opals were discovered in Australia in 1870. Other sources for gem quality opals are India, Brazil, Honduras, Hungary, Slovakia, France, Japan, New Zealand, and the United States (New Mexico, Nevada, Oregon, Idaho). The first synthetic opal, Gilson Stone, was made by Pierre Gilson in the 1960s. It is composed of laminated glass and bits of foil that produced a full spectrum of colors arrayed in a linear fashion. Slocum stone (Opal-essence) is one of the best types of imitation opal.


Synonyms and Related Terms

white opal; light opal; common opal; black opal; fire opal; girasol; harlequin opal; lechosa opal; wood opal; resin opal; hyalite; Muller's glass; milk opal; cachalong; ópalo (Esp.); opale (Fr., It.); Opal (Deut.); opala (Port.); Gilson opal; Slocum stone; Opal-essence; opaal (Ned.)

Other Properties

Massive with no crystalline structure. Cleavage=none Luster = vitreous to pearly. Fracture = conchoidal. Streak = white

Iridescent. Fluoresce=usually white or pale green, some samples phosphoresce

Composition SiO2 - nH2O
Mohs Hardness 5.5 - 6.5
Density 1.98-2.25
Refractive Index 1.43-1.46

Hazards and Safety

Chronic inhalation may cause lung damage

Dehydration may cause surface cracking

Additional Information

Mineralogy Database: Opal


Properties of Common Gemstones

Sources Checked for Data in Record

  • Jack Odgen, Jewellery of the Ancient World, Rizzoli International Publications Inc., New York City, 1982
  • A.Lucas, J.R.Harris, Ancient Egyptian Materials and Industries, Edward Arnold Publishers Ltd., London, 4th edition, 1962
  • Yasukazu Suwa, Gemstones: Quality and Value, Volume 1, Sekai Bunka Publishing Inc., Tokyo, 1999 Comment: RI=1.450; Specific gravity=2.15
  • Michael O'Donoghue and Louise Joyner, Identification of Gemstones, Butterworth-Heinemann, Oxford, 2003
  • C.W.Chesterman, K.E.Lowe, Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf, New York, 1979
  • Van Nostrand's Scientific Encyclopedia, Douglas M. Considine (ed.), Van Nostrand Reinhold, New York, 1976
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998
  • Michael McCann, Artist Beware, Watson-Guptill Publications, New York City, 1979
  • CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: density=2.2