Any of a group of fine-grained, laminated sedimentary rocks consisting of silt- and clay-sized particles. Shale constitutes roughly 60% of the sedimentary rock in the Earth's crust. Shales are commercially important, particularly in the ceramics industry. They are a valuable raw material for tile, brick, and pottery and constitute a major source of alumina for portland cement. In addition, advances in recovery methods may one day make oil shale a practical source for liquid petroleum.