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.
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.
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).
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.
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.