Large quantities of rubies—both rough and faceted—from a commercially important new source in Myanmar (Burma) have been available on the Bangkok market since 1992. The ruby crystals from the Mong Hsu marble deposit have dipyramidal to barrel-shaped habits and reveal dark violet to almost black “cores” and red “rims.” With heat treatment, which removes their blue color component, the cores become intense red. The rubies grew under varying conditions in complex growth sequences. The color distribution between cores and rims is related to a different incorporation of chromium and/or titanium during crystal growth. Gemological, microscopic, chemical, and spectroscopic properties presented here permit the separation of faceted Mong Hsu rubies from their synthetic and other natural counterparts. Problems arising from artificial fracture fillings are also addressed.