Benitoite
Rare and desirable, gem quality benitoite comes from just one location, the Diablo Mountain range of San Benito County, California. In fact, California named benitoite as its state gem in 1985, citing its beauty, rarity, and unique occurrence there. Benitoite is a silicate of barium and titanium. It's chemically similar to sphene. Barium abundance is estimated to be less than 0.05% in the earth’s crust. The extremely rarity of this gemstone makes it one of the most valuable gemstones in the world as it’s considered a collector’s stone.
Benitoite’s color is frequently likened to sapphire’s, it’s a transparent stone that can be blue or violetish blue. It often has light blue, colorless, or white zoning. It also has strong blue and colorless pleochroism. In very rare circumstances the gem can be pink. Benitoite’s high dispersion value is approximately the same as diamond’s, but the stone's body color masks its exceptionally strong fire. There are no commercial treatments for benitoite. As about 6.5 on the Mohs scale, benitoite is hard enough for most jewelry uses, but it needs special care.
Most faceted stones are less than 1.00 carat in size. Gems above 1.0 carat are rare and more valuable, and those above 3.0 carat are extremely rare. When faceted, it’s used primarily in unique high-end jewelry. Natural benitoite usually contains natural inclusions such as fingerprints, mineral inclusions, two-phase inclusions, fracture, fibrous inclusions and dark crystals.
Source: Gemological Institute of America