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

Abstract

The previous two articles in this series de- scribed the origins of color in gems that derive from isolated structures of atomic dimensions-an atom (chromium in emer- ald), 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 &a- monds, orby 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 inicro- scopic dimension, but considerably larger than the clusters of a few atoms previ- ously discussed. ABOUT