The best-known and generally least expensive variety of chalcedony is carnelian. It ranges in color from yellow-orange to rich, near reddish orange, to orangey brown, and varies from semi-opaque to highly translucent. Iron is the source of its color and as a result it can be easily heat treated, (even by the sun’s heat alone,) to darken red tones as the iron is oxidized. You should assume, unless informed otherwise, that any piece of carnelian has been enhanced in this way.