Many deposits of gem corundum are associated with alkali basalt. Best known are the occurrences in Southeast Asia (especially Thailand) and Australia, which are responsible for much of the sapphire and fancy sapphire on the world market today. Yet alkali basalts are relatively uncommon, and corundum deposits are found only in those alkali basalts that occur in association with subduction zones involving ocean-continent plate collisions. This article offers a new hypothesis for the relationship between this rock type and gem corundum. It suggests that gem corundum is formed when certain rocks—such as shales, laterites, or bauxites, with aluminum-rich minerals—are subducted to depths of about 50 km, where metamorphism converts the minerals into corundum and other products that the alkali basalts later carry to the surface. The potential for new deposits is considered.