Benitoite Info

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General

Benitoite is a rare, sapphire-blue (very rarely pink) mineral, a silicate which contains barium, cesium, fluorine, iron, lithium, manganese, niobium, and titanium, almost exclusively mined at the serpentine body of the New Idria Mining District, near the San Benito River in San Benito County, California, USA.
It is hard enough (Mohs scale 6.5) to be used in jewelry but still rather unknown to the public. Faceted benitoite gemstones weight almost always less than 1 ct. A faceted, clear benitoite with a good color, over 0.5 ct, is also rare.

Formation

Benitoite, Neptunite
and Joaquinite

Gemmy benitoite occurs only in the most highly mineralized (more than 150 semiprecious minerals and gemstones) area of California, the New Idria Mining District in the San Benito County / USA. There it can be found at the southern end or the "Diablo Range", a belt of mountains which consists of basalt, chert, graywacke, shale, serpentine, and Cretaceous and Tertiary sandstones. Benitoite crystals were formed from hydrothermal solutions in natrolite veins, containing relatively high concentrations of unusual elements like barium, titanium, fluorine, iron etc. It mainly occurs along with these minerals: Albite, joaquinite, natrolite, neptunite, and serpentine. Natrolite, the last mineral which was formed at the benitoite deposit, coated all minerals of that deposit. Hence the natrolite has to be removed with acid (HCl) to expose the benitoite crystals.

Resources

Clear Creek *

Benitoite Plaque
at Coalinga Rd. *

The most famous (because of its most valuable crystals) benitoite mine is the "Benitoite Gem Mine" (also known as "Benitoite Mine" and "Dallas Gem Mine"), now (2007 / 2008) owned by Coalinga resident Dave Schreiner. Dave is working to allow people to collect on the mine site under a fee dig system.
Four other mines are located within a 10 km radius at the Clear Creek: The "Junnila Mine", about 8 km northwest of the "Benitoite Gem Mine", the "Numero Uno Mine", the "Victor Claim" ("Victor Mine"), and the "Santa Rita Peak Property".
Besides the San Benito County, benitoite of lower quality is found only in Arkansas ("Diamond Jo Quarry" / Magnet Cove / Hot Spring County / Arkansas / USA), Big Creek (Sierra Nevada / Fresno County), Japan (Ohmi / Itoigawa / Niigata Prefecture / Chubu Region / Honshu Island), and Broken Hill (New South Wales / Australia).

* Photo by kind permission of © www.schweich.com, Tom Schweich.
* Photo by kind permission of © www.trinityminerals.com, John Veevaert.

Color

The sapphire blue (with a hint of violet) color of benitoite (sometimes called "blue diamond") is probably based on traces of iron. Very rarely clear, white, greenish-gray, pink, orange, red, and maroon crystals (with inclusions of other minerals like neptunite) are found. Benitoite has a higher birefringence than diamond. Because of its strong dichroism benitoite can show no color or blue or violet. Under short wave UV light it fluoresces bright blue, under long wave UV light some crystals show a reddish color. Benitoite's dispersion can cause a twinkle with flashes of red and green.

Quality

Large, faceted benitoites (over 1 ct) are very rare. They usually show a rich blue. Crystals can be cut into gems, as striking and flawless as the best sapphires. Some purchasers prefer a darker blue, others prefer a brighter stone in which dispersion is more evident. No treatments or enhancements are known for Benitoite.
Similar to sapphire, ruby and other well-known gemstones large, rich blue benitoite is very expensive if it is eyeclean or even better. Very light blue or dark blue stones are less valuable. In contrary to most other gemstones the cut is less important because one always tries to achieve the largest faceted stone, despite of windows and phantasy cuts.

Interesting links
Benitoite Mine.com

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