Geologists call benitoite a ditrigonal dipyramidal barium titanium cyclosilicate. It’s a complex way of referring to one of the rarest minerals on earth, a vivid blue gemstone worth between $4,000-$6,000 per carat.
The mineral itself can be found in Montana, Australia and Japan, but gem quality benitoite occurs only in one mine, which is in San Benito County. Named after the county it was found in, benitoite is California’s state gemstone.
Benitoite is formed at the top of lava tubes, like diamonds. The phrase “ditrigonal dipyramidal” refers to its unusual six-pointed formation. The shape gives it a higher light dispersion, or “fire,” than a diamond.
“The crystal formation of benitoite is a previously unknown shape that makes it a rarity,” said Kammie Osborn, owner of Jan’s Rock Shop in San Juan Bautista. “It also fluoresces, which makes it easy to find at night using ultraviolet light. It is a beautiful blue, and you can easily see why they chose it to be the state gemstone.”
Benitoite measures a hardness of 6.5 on the Mohs scale, a standard gauge used by geologists. In comparison, diamonds are the hardest mineral, rating a 10 on the scale.
“Benitoite is very soft, so it is not ideal for rings,” Osborn said. “It sparkles and refracts very nicely. But unlike a diamond it fractures very easily and is more suited to pendants or pins.”
While famous for its extreme rarity, the public now has a year-round opportunity to sort through tailings brought from the mine to the Benitoite Mining Company, where they can perhaps find their own bits of the mineral.
Benitoite was discovered in 1907 by prospector James Marshall Crouch, who had been staked $25 by mining speculators Roderick Dallas and Thomas Sanders to explore the Diablo Range in the New Idria mining district of San Benito County.
One morning, near the headwaters of the San Benito River, Crouch stumbled across a hillside littered with what he initially thought were blue diamonds or sapphires. Samples were sent to Associate Professor George Louderback at the UC-Berkeley for analysis.
On July 31, 1907, he published his findings in the university’s Bulletin of the Department of Geology, saying “the progress of the investigation has shown that it is a new mineral species” and, with that, the gemstone made newspapers across the country. The news caught the eye of collectors and jewelers worldwide, with one of the first large orders being placed by the Tiffany & Co. jewellers.
Geology teacher Suzie Harlow has an extensive collection of benitoite that she inherited from her father, mineral collector Art Pawson. Pawson had a rare opportunity to visit the mine in 1937 during a period when it was inactive.
Harlow said her father came back with some fine specimens that she regularly displays at rock and mineral shows and occasionally at Jan’s Rock Shop for fourth-grade students in town on visits to Mission San Juan Bautista.
“The most joy I get from my dad’s collection is letting people see this stone they have never seen before,” she said. “I have a passion for getting kids excited about rocks and minerals, and showing this one is an excellent way to have them connect to the Earth and geology.”
Over the years, the mine has gone through many hands. Dave Schreiner, the current owner, bought it in 2005.
“People write stories about the mine who have never talked to me or been out there,” Schreiner said. “There are a lot of misconceptions like, ‘I heard the mine was closed.’ The mine is still active, but we really have no idea how much benitoite is left.”
He said the uniqueness of the mine has made it a target for trespassers. Break-ins have led to the mine entrance being locked up, and security cameras monitor the surrounding area. It is also a habitat for rattlesnakes and poisonous scorpions.
Since the public is not allowed access to the mine, in 2015 Schreiner opened the Benitoite Mining Company, located across the Fresno county line in Coalinga, where people can search for the small gems themselves.
“We wanted to give everyone the opportunity to find some benitoite,” he said. “People can spend the day trying to find something, and as gem mines go, the material is pretty rich. We have people who come every weekend, and it has really turned into a fun community event.”
The site is open every Saturday for up to 40 visitors. Admission is $100 per adult and $50 for children under 12 years old.
Shovels, screened boxes, washing tables, and safety equipment such as glasses and gloves are provided. Experts on site help visitors identify minerals, and a room with ultraviolet light makes finding benitoite easier.
“There is no guarantee, but the chances are good someone will find some little pieces or other minerals like neptunite,” he said. “It is pretty demanding work, though, so people should be ready for that.”
Schreiner sells gemstone-quality benitoite at prices ranging from 0.17-carat marquise-cut stones for $77 to a 2.93-carat oval-cut stone for $13,695. Jan’s Rock Shop sells specimens such as crystals still set in white natrolite; the mineral benitoite is always found within.
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