An Update on Color in Gems. Part 2: Colors Involving Multiple Atoms and Color Centers

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 irradiation (natural or artificial), known collectively as "color centers" (e.g., green diamond).