A dense siliceous rock having the texture, dull luster, hardness, conchoidal fracture, and general appearance of unglazed porcelain; it is less hard, dense, and vitreous than chert. The term has been used for: an impure chert, in part argillaceous; an indurated or baked clay or shale often found in the roof or floor of a burned-out coal seam; and a finegrained, acidic tuff compacted by secondary silica. Etymol: Italian porcellana, porcelain. Also spelled: porcelanite; porcelainite.