Mineral inclusions and coexisting minerals of gem corundum from the Thailand area analysed by SEM-EDS in this study can be divided into sapphire and ruby suites. The sapphire suite includes alkali feldspar (sanidine), nepheline, zircon, hercynite-spinel series and magnetite-hercynite series. The ruby suite includes garnet (pyrope), high-Al diopside (‘fassaite’) and sapphirine. The mineral chemistry and other data suggest that Thai rubies could have crystallized in high-pressure metamorphic rock of ultramafic/mafic composition. In contrast, Thai area sapphires could have crystallized in high-grade metamorphic (gneissic) rock or from magmas of highly alkaline composition, located at shallower levels in the lithosphere than those hosting the Thai rubies.