Benitoite is a very rare blue crystal that was designated the official state gem of California USA by Governor George Deukmejian in 1985. It is a barium titanium silicate mineral with the chemical formula BaTiSi3O9. Benitoite was first found in the early 20th century in San Benito, California (hence the name) - as well as in Arkansas (USA) and Japan. Of these, only the Californian benitoite has so far been of gem quality. 
Benitoite (blue) with neptunite (black) and joaquinite.
The technical name for benitoite's crystal system is Hexagonal - Ditrigonal Dipyramidal.  What this means essentially is that Benitoite crystals typically have the form of being "triangular with the corners removed" when viewed from "above", and tabular / stepped when viewed from the sides. It's a very attractive and unusual crystal shape.
Benitoite is highly prized by both gem collectors and jewellers. It has the hardness (6-6.5 on the Mohs scale) to be suitable for cutting into gemstones. Its rarity, together with the shutdown of the benitoite supply (see below) has pushed the price of high quality benitoites to stratospheric levels - and just now I saw a 1.67 carat Benitoite with fantastic color and clarity (as far as can be seen from a web page!) for sale for $5,475 - an astonishing $3278 per carat; higher than the price of many diamonds, rubies, sapphire, emeralds or tanzanites!
It's said that Benitoites of over 1 carat are very rare. It's interesting to note that the sales page mentioned above said that the mine is no longer open and that Benitoite of this quality is no longer being mined. This would mean that any discovery of gem quality benitoite would be an incredible strike. Looking into this further, I found some very interesting information indicating that gem quality benitoite rough may (just) be obtainable. But there are a few twists to the tale...
Californian benitoite, the only gem quality benitoite yet found in the world, comes from the Benitoite Gem Mine, near the headwaters of the San Benito River, 1 mile south of Santa Rita Peak, San Benito County, California.  Small amounts of benitoites have also come from mines in the nearby area. 
Gem Quality Benitoite - How Much Is Available?
In 2007, a 34.4 carat gem quality Benitoite rough was found, yielding 4 cut stones including a flawless 8.06 carat benitoite, the third largest in the world and valued at $500,000.  However some sources online state that the mine has yielded the best of what it has to offer and that the amount of gem quality benitoite remaining is insignificant.
On May 1st 2008, due to health concerns over exposure to asbestos (there are old asbestos mines in the area), the EPA (Environmental Protection Agency) the BLM (Bureau of Land Management) somewhat controversially (  ) closed off 31,000 acres (imagine a square with a side of nearly 7 miles) surrounding the Benitoite Gem Mine from general public access for a two year study.  
In May 2009, the mine's owner opened a "fee dig area" (a place where the public can pay a fee to hunt through material) for benitoite. The public is not allowed to visit the area unless in a vehicle owned by the mine. He brings material out from the mine for the public to work through. It's said that in the past year, over 4 pieces were found that yielded cut stones over 1 carat. So, statements that the mine is all worked out, look as though they may not be strictly true; also, as the original miners may have worked rapidly through high volumes of material, there may still be gems to be found in material that was once discarded. It's also possible that there may be other undiscovered veins in the area.  
One of the fascinating qualities of Benitoite is that it fluoresces under blacklight (shortwave UV). The Californian Benitoite mine has a UV darkroom available for gem hunters - and the daily fee is $70. 
Benitoite (blue) with neptunite (black) and natrolite (white).
Benitoite (faceted, from California, USA. 0.25 carat)
Image © supplied by Woodmansee* Gems
Benitoite - Sources Referenced:
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