An Update on Color in Gems. Part 3: 

Colors Caused by Band Gaps and Physical Phenomena

Emmanuel Fristch
Gemological Institute of America
Santa Monica, California, USA

George R. Rossman
 California Institute of Technolgy, Pasadena, CA, USA


This is the third part in a three-part series on the origin of color in gem materials. This article 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..

Gems and Gemology 24, 81-102.