New Idria's Geology

The geology of the Idria region is often described as a (Franciscan Formation) complex distribution of sandstone, shale and jasper intermingled with metamorphic rocks with a large serpentine intrusion area. The region is generally divided between serpentine and non-serpentine area. Serpentine is California's state mineral. Because the serpentine soil is nutrient-poor, it does not support the diverse vegetation found in the non-serpentine area. For this reason, the serpentine area contains large barren spaces that make it very popular with motorized dirt bike recreation. New Idria is located in a non-serpentine area of this region.

As a mining district, the region is well known for its mineral deposits. In fact, this is one of the most highly mineralized areas in California and as such, it attracts many recreational gem and mineral collectors. They are drawn to the area by the presence of more than 150 semiprecious mineral and gemstones. Some minerals, such as benitoite, are extremely rare, and this is practically the only place where they may be found. This is the only place in the world where gem-quality benitoite, the State's Gemstone, is found. Except for the Benitoite Gem Mine and the LABSCO Bentonite Mine, all other mining in the region has ceased.

Benitoite and bentonite have similar spelling, but they are totally unrelated minerals. Benitoite (BaTiSi₃O₉, barium titanium silicate) is one of the rarest gemstones on earth and it is named for San Benito County, the only place in the world where gem quality benitoite is found. Bentonite is a clay mineral that comes from volcanic ash. Bentonite is used in oil, gas, and water well drilling, metal-casting, environmental construction and remediation, hazardous waste treatment, cat litter, cosmetics and pharmaceuticals, as well as many other industrial and consumer related products. Bentonite is named after Benton Formation in eastern Wyoming's Rock Creek area.