An Update on Color in Gems. Part 2: 

Colors Involving Multiple Atoms and Color Centers

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

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


This is the second part in a three-part series on the origin of color in gem materials. This article discusses colors produced by (1) processes that involve multiple atoms (e.g., blue sapphire, organic products), and (2) a variety of defect structures that are generally created by irradiatin (natural or artificial), known collectively as "color centers" (e.g., green diamond).

Gems and Gemology 24, 3-15.