Feature Gems & Gemology, Fall 1997, Vol. 33, No. 3

Benitoite from the New Idria District, San Benito County, California

Commercial quantities of gem-quality benitoite are known from a single location in the world, the Benitoite Gem mine in the New Idria district of San Benito County, California. A barium titanium silicate, benitoite is typically colorless to blue, and is noteworthy for its high refractive indices, moderate birefringence, and strong dispersion. Benitoite occurs in altered blue schist within serpentinite. The gem crystals formed within fractures as a result of the alteration of blue schist by hydrothermal fluids derived from the regional metamorphism of the serpentinite. An extension of the historic deposit at the Benitoite Gem mine was discovered in the spring of 1997, which should contribute to continued stability in price and supply. The gemological properties of benitoite readily identify it from similar-appearing gems.