Feature Gems & Gemology, Summer 1988, Vol. 24, No. 2

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 (Fe2 +-0-Fe3 + units in cordierite). The final part of this series is concerned with colors explained by band theory, such as canary yellow diamonds, or by 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 microscopic dimension, but considerably larger than the clusters of a few atoms previously discussed.