Plate tectonic gemstones

The gemstones jadeite and ruby generally form as a result of the plate tectonic processes subduction and collision. Jade made of jadeite (jadeitite) forms when supercritical fl uids released from subducting oceanic crust condense in the overlying mantle wedge, 20-120 km deep in the Earth. Jadeitite deposits thus mark the location of exhumed fossil subduction zones. Ruby, the red gem variety of corundum, forms during amphibolite- and granulite-facies metamorphism or melting of mixed Al-rich and Si-poor protoliths, 10-40 km deep in the crust. Suitable conditions generally exist where passive-margin carbonates and shales are involved in continental collision. Most ruby deposits formed during Ediacaran-Cambrian (ca. 550 Ma) collisions that produced the East African-Antarctic orogen and the supercontinent Gondwana, or during Cenozoic collisions in south Asia. Ruby is thus a robust indicator of continental collision. As a result of these diagnostic properties, we propose the term "plate tectonic gemstones" (PTGs) for jadeitite and ruby. The PTGs are a new type of petrotectonic indicator that are mostly found in Neoproterozoic and younger rocks. The PTGs as petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion, although the possibility of destruction via retrogression needs to be further assessed. Recognition of the PTGs links modern concepts of plate tectonics to economic gemstone deposits and ancient concepts of beauty, and may aid in exploration for new deposits.