Gems: A Lively Guide for the Casual Collector
“Unlike most mineral groups, which are colored by one or two trace elements that reside in the crystal, the tourmaline mineral group is home to a wide variety of these ‘invaders.’ This is why the tourmaline family is found in a rainbow of colors considered by experts to span the widest and most colorful gem spectrum of all… Tourmalines are often multicolored, and many varieties are strongly pleochroic, showing different colors when viewed from different angles,”
- Gems: A Lively Guide for the Casual Collector by Daniel J. Dennis Jr.