Tourmaline: A Geologic DVD

Tourmaline is an eye-catching mineral, but even more importantly, it has played a significant role in the evolution of scientific thought and, more recently, has been recognized as a medium for recording geologic information, not unlike a DVD. With its plethora of chemical constituents, its wide range of stability from conditions near the Earth's surface to the pressures and temperatures of the upper mantle, and its extremely low rates of volume diffusion, tourmaline can acquire a chemical signature from the rock in which it develops and can retain that signature through geologic time. As a source as well as a sink for boron, tourmaline is nature's boron recorder. Tourmaline can be used as a geothermometer, provenance indicator, fluid-composition recorder, and geochronometer. Although long prized as a gemstone, tourmaline is clearly more than meets the eye.