A small stone, especially one worn smooth by erosion. Clear colorless quartz; rock crystal. A lens made of such quartz. Geology A rock fragment between 4 and 64 millimeters (0.16 and 2.51 inches) in diameter, especially one that has been naturally rounded. An irregularly rough, grainy surface, as on leather or paper. To pave with pebbles. To impart an irregularly rough, grainy surface to (leather or paper). To pelt with pebbles.