A fine-grained hard stone, a variety of the silica mineral quartz. A form of chert, it is found in a variety of milky or grayish colors with distinctive parallel bands of contrasting color. In antiquity, chalcedony was the stone most used by the gem engraver for beads, seals, and sometimes as a substitute for flint. The agate, carnelian, jasper, and onyx are some of the varieties still cut and polished as ornamental stones.