Benitoite is a rare mineral first discovered in California in 1907 by James Couch. At first it was believed to be sapphire, but samples were sent to the University of California, Berkeley where geologist George Louderback identified it as a mineral new to science. He named it benitoite after its type location near the headwaters of the San Benito River in San Benito County. San Benito County is located in the coastal range mountains of Northern California, south of San Jose and inland from Monterey Bay.
By composition, benitoite is blue barium titanium silicate, found in hydrothermally altered serpentinite. It is not only very rare, but it tends to occur with other unusual minerals, including natrolite, neptunite, joaquinite, serpentine and albite.
Though benitoite has been found in a few other locations in the world (in Arkansas, USA and in Japan and Australia), so far, the only gem-quality specimens have been found in California. In 1985 benitoite was designated as the official state gem of California.
Mainly small crystals of benitoite - weighing less than 1 carat - have been found in gem quality. Though benitoite is not particularly hard (6 to 6.5 on the Mohs scale), it has an unusually high refractive index of 1.757 to 1.804, higher than sapphire. It also has dispersion slightly greater than diamond. So a gem-quality benitoite can display impressive brilliance and fire.
Benitoite also has a very unusual crystal formation. It crystallizes in the ditrigonal-dipyramidal class of the hexagonal crystal system.
Since gem-quality benitoite is so rare, benitoite is mainly of interest to mineral collectors, especially specimens which show off this mineral's unique crystals, or specimens in which benitoite occurs with its commonly associated minerals.
The Benitoite Gem Mine in San Benito County ceased commercial mining operations in 2006. But the mine is open to mineral hobbyists and tourists, who pay a daily fee to screen material from the mine.
- First Published: January-19-2010
- Last Updated: August-17-2017
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