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Smithsonian National Museum of Natural History

Fire Opal

Photograph of a hexagonal faceted opal (NMNH G3593) with reflection
Photo by Chip Clark. Click to zoom.
Catalog Number NMNH G5393-00
Locality Jalisco, Mexico
Weight 29.85 ct

Opal is a noncrystalline hydrated form of silica (silica and oxygen), forming when silica slowly settles out of a dilute water solution. Fire opals are transparent to semi-transparent, resembling gelatin, with red, orange, or yellow body color, with or without play-of-color. Fire opal gets its name from its reddish orange body color, which is caused by inclusions of iron oxides. They are also sometimes called Mexican opals because most of the best fire opals are found in Mexico. Because of their transparency, fire opals are commonly faceted, as seen here.



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