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Oxygen isotopic compositions of rubies and sapphires from 106 deposits worldwide, as well as heated natural corundum, have been measured in this study. Artificially heated corundums have the same oxygen isotopic composition as unheated material. The [sup.18]O/[sup.16]O ratio of natural corundum is a good indicator of its geological environment of formation. The consistently restricted [delta][sup.18]O range found for each type of deposit is explained by host-rock buffering during fluid-rock interaction. The [sup.18]O constrains the geological source of the major type of gem-quality rubies sold on the market and brings new insight to gems found in placers. High-quality blue sapphires from Kashmir, Andranondambo, and Sri Lanka have specific oxygen isotopic ranges, but they overlap those of Mogok in Myanmar. Combined with traditional gemology techniques, oxygen isotope analysis will contribute toward defining the origin of some commercial high-value blue sapphires, especially from Kashmir. Keywords: ruby, sapphire, oxygen isotopes, placer, geographic origin, gemological applications.