Serpentinite

Description
A picture does not quite do it, for it is the “feel” of this fault rock that is its fundamental characteristic. The “feel” is slippery and ultra smooth, for it is fundamentally talc, as in talcum powder. The serpentinite was produced through shearing and metamorphism of ultramafic rocks at great depth, in the deep reaches of the major shear zone that the San Andreas fault represents. Hydrothermal (hot water) alteration during faulting transformed the serpentinite, at least in part, to talc. Some geologists believe that the creeping segment of the San Andreas fault (i.e., moving all the time, as opposed to earthquake by earthquake) can be attributed in some locations to the presence of slippery talc.
Whereabouts

I collected this specimen in 2009 at a roadcut outcrop along the Shoreline Highway, a few kilometers southeast of Stinson Beach, California.  My wife and I were driving north from San Francisco to visit Point Reyes National Seashore, with its earthquake-trail walk that celebrates the observations of G. K. Gilbert following the Great 1906 Earthquake.

Age & Formation
Serpentinite from fault zone along San Andreas fault, California.