This month's stone profile:

 

Benitoite

 
Refractive Index:  1.76-1.80 Crystal Structure: Hexagonal
 

 

 
Hardness:  6 1/2 Specific Gravity:  3.67
 

 

 
Chemical Composition:  Barium titanium silicate Occurrences:  San Benito Co., California

Discovered nearly a century ago, Benitoite has remained a single source gemstone.  Rarer still is the fact that the single source is in the U.S.  Gem quality Benitoite is only found in California, with one major (called simply the Benitoite Gem Mine) and several minor occurrences (including the Junnila Claim), all in San Benito County.  Chemically related to last month's featured stone, Sphene, it's top color is a medium dark, saturated blue to slightly violetish-blue, though lighter tones down to colorless do exist.  Benitoite has a high dispersion, though this can be a trade off as the more valued darker blues tend to mask the dispersion.

Valued also for it's crystal morphology, Benitoite rough is usually small and stones over a carat take a dramatic jump in rarity.  Stones less than one carat are estimated to account for nearly 90% of all faceted material.  A 15.42ct. stone in a private collection is currently the largest faceted example of Benitoite.  Usually untreated, a small number of stones respond to heating by changing, wholly or partially, to an orange color.

Despite its rarity, the last decade or so has seen steady production of Benitoite and prices have remained relatively stable. Perennially popular with collectors and a viable jewelry stone, we would appear to be in a buyer's market for this material.  Though Benitoite could hardly be described as plentiful, it certainly is as available now as at any point during the last  hundred years.