Benitoite

Benitoite
Specific gravity		3.65
Optical properties	Uniaxial (+)
Refractive index	nω = 1.756 - 1.757 nε = 1.802 - 1.804
Birefringence	δ = 0.046
Pleochroism	O = colorless; E = purple, indigo, greenish blue
Dispersion	0.036–0.046[2]
Solubility	Insoluble: HCl, H2SO4 Soluble: HF
Other characteristics	Blue fluorescence under SW UV; intense blue cathodoluminescence
References	[3][4][5]

Benitoite (

, appearing bright blue to bluish white in color. The more rarely seen clear to white benitoite crystals fluoresce red under long-wave UV light.

It was discovered in 1907 by prospector James M. Couch in the

Benitoite occurs in a number of isolated locations globally, but gemstone quality material has only been found in California at the Benito Gem Mine where it was first discovered. It has been correctly identified in Montana, Arkansas, Japan, and Australia although they formed under slightly different conditions and only grow large enough to be considered an accessory mineral. In 1985 benitoite was named as the official state gem of California.^[8][9]

Non-gem crystals of benitoite can have a very rare, six-pointed twinned form.^[10]

Associated minerals and locations

Benitoite typically occurs with an unusual set of minerals, along with minerals that make up its host rock. Frequently associated minerals include:

.

Benitoite is a rare mineral found in very few locations including San Benito County, California, Japan and Arkansas. In the San Benito occurrence, it is found in natrolite veins within glaucophane schist within a serpentinite body. In Japan, the mineral occurs in a magnesio-riebeckite-quartz-phlogopite-albite dike cutting a serpentinite body.^[5]

• 
  Blue benitoite crystals on white natrolite, Dallas Gem Mine, San Benito Co., California, US
• 
  Benitoite crystals under UV light

References