Benitoite: A rare gemstone

by Lucas Joel
Monday, January 11, 2016

Gem-quality specimens of benitoite, California's state gemstone, are found only in San Benito County where Pinnacles National Park is located. Credit: Didier Descouens, CC BY-SA 4.0.

The tectonic forces that formed Pinnacles also helped form one of the world’s rarest gemstones: benitoite. Gem-quality specimens are only found in San Benito County where Pinnacles National Park is located. Benitoite, which is California’s state gemstone, has a deep azure color, a pyramidal crystal habit, and a light-blue fluorescence under ultraviolet light.

The benitoite in San Benito County is often found with serpentine, which comprises the bulk of California’s state rock, serpentinite. The serpentine formed along the Farallon Plate subduction zone. Benitoite was originally thought to be sapphire, but in 1907 it was identified by George D. Lauderback of the University of California at Berkeley as a new mineral species. Lauderback named the new gem after the San Benito River, where benitoite gems occur.

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