A family of colored quartz, usually with a milky or waxy
appearance, including carnelian, agate, cats eye and jasper. It
is common in petrified wood, and fills open spaces in many
volcanic rocks. Colourful specimens are commonly called agate,
and if suitable for particular carvings can also be called onyx.
Sometimes spelled 'calcedony' |Etymology: Latin. calcedonius,
from Greek khalkedon, a mystical stone|