volcanic tuff

A type of sedimentary rock that is characterized by its layered, grainy appearance. Tuff consists of a mix of fine-grained rock and condensed volcanic ash that have been meshed and pressed together after a volcanic eruption. Tuff often appears porous, and is subject to changes in formation over time as a result of exposure to wind, water, or ice.

Note that tuff is the correct spelling of this type of rock, although in the novel People of Darkness it is spelled volcanic tuft.