gems, geology, East Africa, Gondwana, Detrital zircons, Tectonics, Cambrian Explosion, John Saul, Penny Lane, ruby, Kenya

A puzzle, almost as old as the modern science of gemology itself, concerns metamorphic minerals such as garnet and corundum that normally crystallize in an opaque, stony manner. Under rare circumstances, however, such minerals crystallize as transparent gems. Why should that be? An area in the Kenyan-Tanzanian border region provides clues to why, where, and when gem-quality crystallization occurs. Higher than usual temperature at depth permits metamorphic minerals to crystallize at shallower levels. and thus with less than usual constraining pressure, hence potentially as transparent gems. A source of short-lived excess heat in the Penny Lane area (Kenyan-Tanzanian border) is furnished by friction produced by nearby thrust faults.