A fine-grained fibrous variety of quartz, silicon dioxide. The individual fibers that compose the mineral aggregate usually are visible only under the microscope. Subvarieties of chalcedony recognized on the basis of color differences (induced by impurities), some valued since ancient times as semiprecious gem materials, include carnelian (translucent, deep flesh red to clear red in color), sard (orange-brown to reddish brown), and chrysoprase (apple green). Chalcedony sometimes contains dendritic enclosures resembling plants or trees. Major kinds of impurities that give color to chalcedony are iron oxides (carnelian and sard), nickel (chrysoprase), and manganese.