clays, argillaceous schists, clay marls, and other rocks altered through the action of underground fires in coal seams into dense, porcelaneous rocks, which may be white, pink, mottled, or dark. Porcellanite also contains mullite, cordierite, wollastonite, and other minerals. It is formed at temperatures reaching 1,300°C. Porcellanite is also formed when waste piles burn and sometimes assumes a slaglike appearance.

Porcellanite is used as a building material, namely ballast; when pulverized, it acquires binding properties and is used as a cement additive. In the USSR, it is mined in the Angren River valley in Uzbekistan, where kaolin clays between coal seams have been altered. The term “porcellanite” is common in the United States, where it is applied to an impure variety of a siliceous rock, usually opalescent, that has the texture and luster of unglazed porcelain.