An Update on Color in Gems. Part 3: Colors Caused By Band Gaps and Physical Phenomena

The previous two articles in this series described the origins of color in gems that derive from isolated structures of atomic dimensions-an atom (chromium in emerald), a small molecule (the carbonate group in Maxixe beryl), or particular groupings of atoms (Fez +-0-Fe:f + units in cordieiite). The final part of this series is concerned with colors explained by band theoiy, such as canary yellow &amonds, o r b y physical optics, such as play-of-color in opal. In the case of band theory, the color-causing entity is the very structure of the entire crystal; in the case of physical phenomena, it is of inicroscopic dimension, but considerably larger than the clusters of a few atoms previously discussed.