GO 340 Gemstones & Gemology

Emporia State University

academic.emporia.edu/abersusa/go340/opal.htm

Opal


Image taken from
The Image.
Opal is a hydrated silica, SiO2(H2O), that is lacking in crystal structure or is amorphous. The hardness is 5.5-6.5 and opal has a low specific gravity at 1.98-2.20. It has a conchoidal fracture and is brittle. Opal contains water, from 3-30%, and this over time will be lost, diminishing the opalescence (Schumann, 1997, p. 150). There are basically three types of opal: precious opal (containing flashes of fire), the orangish-red "fire opal" (named for its color, not flashes of fire), and common opal.
Image taken from
The Image.
Opal's play-of-colors sets it apart from most other gemstones. This colorful phenomenon is caused by the diffraction of light from layers of silica and cristobalite spheres layered in a siliceous jelly, which creates an effect similar to the irridescence of a soap bubble. Opal's quality is judged by the background color and play-of-color display.
Image taken from
The Image.
This display is classified by the particular colors, their intensity, size and evenness of the pattern. White or pale blue backgrounds are most common, with red, orange, and violet flashes most desirable. Blue and green flashes in the play-of-color are most common. Black opal or those with a dark background are the rarest and most valued of all opals. Again, the red and orange flashes are most desirable.

Fire opal, taken
from Gem Hut.
In addition to black and white opals, some have a colorless background, with little or no play-of-colors, and are called jelly or water opal. Fire opal may be yellow, orange,or a reddish background and because it is found in Mexico, it is often called Mexican fire opal. View an image with a great variety of fire opals at the Smithsonian, http://geogallery.si.edu/index.php/en/1001575/fire-opal. It is the only type that is faceted and can be a deep red, through many shades of orange, to yellow. It may have a few flashes but is valued for its color and clarity. "Common opal" is rarely transparent and contains inclusions that may be dendritic or moss-like. Common opal is sometimes used as the backing material for more desirable varieties of more precious opal.
Australia may be the world's most important source of opal, but it is also found in Mexico, Brazil, Hungary, and in the US, Nevada, California, and Idaho. Opal is a low temperature mineral, found in cracks or cavities filled after the host rock was formed or as replacement after fossils or wood. It is usually found in sedimentary rock, such as sandstone or claystone, and igneous rock, such as rhyolite. Mining is usually a small scale operation, with hand-operated machinery and tools.
Opal Doublet, taken
from Gem Hut.

Photo date 10/99
© S.W. Aber
Opal is not a durable gemstone, because of low hardness, brittleness, low heat tolerance, and the fact that it may be ruined by exposure to chemicals. As the water content diminishes, the stone may lose color and crack. Therefore, opal is commonly "assembled," or sandwiched between two pieces of other material often a cap of quartz and base layer of common opal or onyx, to improve on its durability. In addition to assembled doublets and triplets, imitations and synthetics are found. Synthetics came on the market in 1974 and imitations are often a variety of glass called a "Slocum Stone." Chatham produced a synthesized or created opal in 1995.
Opal and tourmaline are the commonly accepted October birthstones. It is usually fashioned in a cabochon cut and set in rings, earrings, pendants, or pins. For more information on opal history and lore, visit CW Jewelers and Jewelry Central. Information on mining Australian opals may be found at Opal Mine:
  • http://opalmine.com/opal-information-2/opal-mine/, Opal Mining
  • http://opalmine.com/opal-information-2/opal-fields-of-australia/, Opal Fields and Map
  • http://opalmine.com/opal-information-2/opal-miners-huts/, Opal Miners' Huts
  • http://opalmine.com/opal-information-2/opal-fields-of-australia/lightning-ridge-opal-mining-video/, Opal Mining at Lightning Ridge http://youtu.be/WGKpLunNKAs
  • http://opalmine.com/opal-information-2/opal-types/, Opal types
  • http://opalmine.com/opal-information-2/opal-picture-guide/, Opal Picture Guide
  • http://opalmine.com/services/opal-valuations/, Opal Values
  • http://opalmine.com/services/opal-photography/, Opal Photography

    Opal is commonly assembled to protect the fragile gem. Opal locations, grading, and cutting information may be viewed at House of Tibara Opals. Opalized fossils and other specimens can be viewed at opal-trader.com/opal-fossils.asp; and www.austopalmines.net.au, Australian Opal Mines. A general coverage worth viewing is at Wikipedia! http://en.wikipedia.org/wiki/Opal (13 September 2012).


  • Image taken from Jewelry
    Central
    .

    Excerpts from a fascinating book, well worth reading:

  • Wise, R. (2003). Opal; Myth Magic and Misconception. WWW URL: http://www.secretsofthegemtrade.com/articles_10.htm. From Secrets of the Gem Trade.


    References

    Additional Sites

    Return to the Syllabus or choose another gemstone below.

    Alexandrite Amber Amethyst Chalcedony Diamond Emerald
    Garnet Jade Malachite Opal Pearl Peridot
    Ruby Sapphire Tanzanite Topaz Tourmaline Turquoise

    This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: esu.abersusie@gmail.com Thanks for visiting! Webpage created: November 15, 2000; last update: September 16, 2012.

    Copyright 1999-2012 Susan Ward Aber. All rights reserved.