Serpentine – State Rock

Serpentine, California state rock

Serpentine rock is apple-green to black and is often mottled with light and dark colored areas. It has a shiny or wax-like appearance and slightly soapy feel. Serpentine is usually fine-grained and compact but may be granular, platy or fibrous. It’s found in central and northern California in the Coast Ranges, Klamath Mountains and Sierra Nevada foothills.

Serpentine rock is primarily composed of one or more of the three magnesium silicate minerals: lizardite, chrysotile and antigorite. Chrysotile often occurs as fibrous veinlets in serpentine. Chrysotile in fibrous form is the most common type of asbestos. Asbestos is a group of silicate minerals that readily separates into thin, strong and flexible fibers that are heat resistant. Lizardite and antigorite don’t form asbestos fibers and instead are plate-like.

Serpentine is metamorphic and/or magnesium-rich igneous rock, most commonly peridotite, from the earth’s mantle. (The mantle is a thick layer of rock just below the earth’s crust.) Peridotite underlying
oceanic crustal rocks was metamorphosed to serpentine in subduction zones that existed at various times in California’s past. (A subduction zone is where ocean crust rocks run into and slide underneath
the edge of a continent.) Because serpentine has a much lower density than peridotite, it rose toward the surface along major regional thrust
faults associated with the subduction zones.

Source: The California Department of Conservation, California Geological Survey

Download this information sheet on the California State Rock by The California Department of Conservation, California Geological Survey