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Smithsonian National Museum of Natural History


Photograph of six Mexican fire opals (NMNH G3968, G3886, G2111, G3964, G1072, and G2028) from the National Gem Collection
Photo by Chip Clark. Click to zoom.
SpecimenCatalog NumberLocalityWeight
OpalNMNH G3968-00Querétaro, Mexico143.23 ct
OpalNMNH G2028-00Querétaro, Mexico21.8 ct
OpalNMNH G1072-00Querétaro, Mexico14.5 ct
OpalNMNH G3886-00Querétaro, Mexico11.5 ct
OpalNMNH G2111-00Querétaro, Mexico21.4 ct
OpalNMNH G2240-00Querétaro, Mexico55.9 ct


Opal is a noncrystalline hydrated form of silica (silicon and oxygen), forming when silica slowly settles out of a dilute water solution. Opals are described according to their transparency and body color. White opals, translucent stones with a play-of-color against a white body color, are the most common. Opals with a vivid play-of-color and a black or other dark body color are called black opals and are highly prized. Fire opals are transparent to semi-transparent, resembling gelatin, with red, orange, or yellow body color, with or without play-of-color. The body color 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. Opals are typically cut as cabochons or polished free-form to best show the play-of-color. This group of opals from Mexico range in weight from 11 to 143 carats.


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