I took part in a field trip last weekend in which, among other things, we looked closely at a large unit of rock that most of us had habitually called chert. In fact, we were told, a lot of it is porcellanite. So I'm pleased to have learned a bit more about it and added it to the sedimentary rock gallery. You may not often have reason to identify it, but it feels good when the name of this obscure rock type comes easily to mind.
Porcellanite of the Claremont Formation Geology Guide photo