This specimen is from the Marin Headlands north of San Francisco, where deep-sea chert is widely exposed. (Because the locality is in the National Park system, I left it where I found it.) It is likely that this former seafloor had at least a sprinkling of manganese nodules on it. If those compounds were mobilized during these rocks' travels in the ancient California subduction zone, this crust would be the result.
Manganese oxides are also a major ingredient in desert varnish.