Gem corundum from placer deposits in the vicinity of Ratnapura, southwestern Ceylon, has been known and exploited for many years, but the parent rock, or source, has never been discovered. Geologic evidence indicated derivation from pegmatite veins. Recently, gem-quality corundum was discovered at the contact of a syenite intrusion with limestone, exposed in a new locality southeast of Colombo. At the contact, the syenite is quartz-free and contains areas of diopside-scapolite rock, probably partially digested limestone, and also a soda-rich scapolite, formerly identified as nepheline. In places along the contact, the limestone has caused desilication to the extent of forming corundum which is strikingly similar in crystal form to the alluvial gem corundum. Poor exposures, due to weathering and jungle cover, have limited the discovery of outcrops, but it is here suggested that much of the gem sapphire has been derived from contact zones of acid intrusions into limestone bands of the khondalite series.