Biaxial Optic Figure in Elbaite Tourmaline.
Photo: E. Skalwold
Although tourmaline is a uniaxial mineral, there can be localized domains of lower symmetry which will manifest a biaxial optic figure. Such anomalous optical properties have been described in many gems including jeremjevite, apatite, quartz, garnet and diamond. Usually noted as "strain," they are caused by changes in chemical composition and/or changes during crystal growth.