Darragh, Peter J. Commonwealth Scientific and Industrial Research Organisation, Wembley, Australia.
Klein, Cornelis Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico.
- Gem classification
- Links to Primary Literature
- Additional Readings
A natural hydrated form of silica. Opal is a relatively common mineral in its nongem form, which is known as common opal and lacks the play of color for which gem, or precious, opal is known. All opal is of relatively simple chemical composition, SiO2 · nH2O, in which SiO2 represents silicon dioxide, and n represents a variable amount of water (H2O) that is housed in the internal structural arrangement of the mineral. The water content generally ranges from about 4 to 10 wt %, but it may be as high as 20 wt %. The hardness of opal on the Mohs hardness scale ranges from 5 to 6, the specific gravity from 2.25 to 1.99, and the refractive index from 1.455 to 1.435. Both specific gravity and refractive index decrease as a function of increasing water content. See also: Hardness scales
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Its dedicated editorial team is led by Sagan Award winner John Rennie. Contributors include more than 9000 highly qualified scientists and 39 Nobel Prize winners.
MORE THAN 8500 articles and Research Reviews covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 17,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information