Benitoite () is a rare blue

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

titanium cyclosilicate, found in hydrothermally altered serpentinite. It forms in low temperature, high pressure environments typical of

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

zones at

convergent plate boundaries A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a p ...

. Benitoite fluoresces under short wave ultraviolet light, 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 San Benito Mountains roughly halfway between San Francisco and Los Angeles. Due to its similar color, Couch originally believed it to be sapphire, a variety of

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the pres ...

. In 1909, a sample was sent to the University of California, Berkeley, where mineralogist Dr. George D. Louderback realized it was a previously unknown mineral. Corundum (sapphire) has a defined Mohs hardness of 9, while benitoite is much softer. He named it ''benitoite'' for its occurrence near the headwaters of the San Benito River in San Benito County, California. 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 Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...

, and

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

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

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: natrolite, neptunite, joaquinite,

serpentine Serpentine may refer to: Shapes * Serpentine shape, a shape resembling a serpent * Serpentine curve, a mathematical curve * Serpentine, a type of riding figure Science and nature * Serpentine subgroup, a group of minerals * Serpentinite, a ...

and albite. Benitoite is a rare mineral found in very few locations including San Benito County, California,

and Arkansas. In the San Benito occurrence, it is found in natrolite

veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...

within glaucophane schist within a serpentinite body. In Japan, the mineral occurs in a magnesio- riebeckite- quartz- phlogopite- albite

dike Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...

cutting a serpentinite body.Handbook of Mineralogy
/ref> File:benitoite09.jpg, Blue benitoite crystals on white natrolite, Dallas Gem Mine, San Benito Co., California, US File:Benitoite, UVL.jpg, Benitoite crystals under UV light

References

{{reflist, 25em Cyclosilicates Gemstones Titanium minerals Hexagonal minerals Minerals in space group 188 Barium minerals Luminescent minerals Geology of San Benito County, California Geology of California Symbols of California