Rocks of Upper Rockridge III: Blueschist

At the uppermost top of Upper Rockridge is Contra Costa Road, where amid the fine homes and gardens is this knocker of real blueschist.

Knockers are the California geologist’s nickname for blocks of resistant rock in Franciscan mélange, the smorgasbord of rock types (chert, basalt, greenstone, serpentinite, etc.) bound by shaly matrix that is common throughout the Coast Ranges. Knockers are too big to be called boulders but too small to be mapped. Anyway, this knocker is a tough stone of a deep indigo color from the high-pressure mineral glaucophane, which if you remember your Greek simply means “blue in appearance.” I couldn’t resist taking home a chip.

Unlike the garnet-mica blueschist of Joaquin Miller Park, this outcrop is almost monomineralic except for some white veins, probably quartz. It gleams like leather in the magnifier, with intricate crenulations and understated foliation—not a real schisty schist, but layered enough to qualify. I’m in love with it.