From the Vikings’ sunstone to a modern piezometric pressure sensor, tourmaline is an intriguing mineral with a new degree of significance. Tourmaline was considered by 18th century physicists as the key to a grand unification theory relating heat, electricity, and magnetism, but new studies define its role as an indicator of Earth’s processes. With its plethora of chemical constituents and its wide stability range, from near-surface conditions to the pressures and temperatures of the mantle, tourmaline has become a valuable mineral for understanding crustal evolution. Tourmaline encapsulates a single-mineral thermometer, a provenance indicator, a fluid-composition recorder, and a geochronometer. Although also prized as a gemstone, tourmaline is clearly more than meets the eye.
