Gem-quality benitoite has been found in exactly one place on Earth, and this is it.
Benitoite is a rare blue barium titanium cyclosilicate that forms in hydrothermally altered serpentinite, under the low-temperature, high-pressure conditions typical of subduction zones at convergent plate boundaries. Webmineral records the environment precisely: compact granular natrolite veins cutting a greyish-green fibrous schist interlayered with serpentine, at the Benitoite Gem mine at the headwaters of the San Benito river, a mile south of Santa Rita Peak in San Benito County.
The discovery is a good story about being wrong first. The prospector James M. Couch found it in 1907 in the San Benito Mountains, southeast of San Jose, and took the blue crystals for sapphire. In 1909 a sample went to the University of California, Berkeley, where the mineralogist George D. Louderback recognised it as a mineral nobody had described. The giveaway was hardness: corundum, which sapphire is, sits at 9 on the Mohs scale, and benitoite is a good deal softer. He named it for the river.
It does occur elsewhere. It has been correctly identified in Montana, Arkansas, Japan and Australia, but those formed under slightly different conditions and the crystals only ever grow large enough to count as an accessory mineral. Gemstone-quality material has only ever come from California, from this mine.
It also does something worth seeing in the dark. Benitoite fluoresces bright blue to bluish white under short-wave ultraviolet, and the rarer clear to white crystals fluoresce red under long-wave UV.
It was named the official state gem of California in 1985.
Benitoite
Other places on the map within a couple of hours.
