Blueschist-facies rocks, like those at the Laytonville quarry I visited last weekend, have minerals rarely seen elsewhere lurking among the common ones. A band of pinkish gneiss contained these arresting rosettes of the iron-bearing mineral stilpnomelane (stilp-NO-melane). It's a phyllosilicate mineral related to the micas, but with extra iron in both divalent and trivalent states.
This was an outcrop that, in accordance with my standard field practice, I left alone. But there was plenty of broken rock lying around from which I collected a piece of highly schistose rock with stilpnomelane layers shining like wrinkled dinosaur skin. Every field trip with a new mineral is an especially good one, but geologizing is like fishingthe worst day doing it beats the best day of working.
Stilpnomelane Geology Guide photo