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Smithsonian National Museum of Natural History

Fire Opal

Fire Opal (NMNH G10585)
Photo by Ken Larsen. Click to zoom.
Catalog Number NMNH G10585-00
Locality Mexico
Weight 23.15 ct

Gift of Anton H. Rosenthal and Ruth E. Ganister in 2008.

Fire opals are transparent to semi-transparent, resembling gelatin, with a red, orange, or yellow body color, with or without play-of-color. The body color is caused by inclusions of iron oxides. Gem opal consists of tiny silica spheres tightly packed together; the voids or spaces between the spheres contain air or water. The play-of-color in opal is due to the orderly arrangement of these spheres acting like a diffraction grating, breaking visible white light in to separate colors. Fire opals are commonly referred to as Mexican opals because most of the finest fire opals are found in Mexico. Opals are typically cut en cabochon or polished free-form to best show the play-of-color. However, due to their transparency, many fire opals are faceted. This fire opal cabochon is semi-transparent with an orange body color and displays beautiful green, yellow, orange and red play-of-color. With magnification, looking inside the opal you can see interesting inclusions: “stalagmite and stalactite” formations that are probably goethite rods, along with what appear to be hematite crystal inclusions. This Mexican fire opal has all the characteristics of a unique and beautiful gemstone and is a wonderful addition to the National Gem Collection. (Close-up images of inclusions by Wimon Manorotkul.)



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