Geology and mineralogy of gemming terrains of Sri Lanka

Gemming in Sri Lanka is carried out in river valleys and on hill slopes underlain by high grade Precambrian metasedimentary rock suites. The principal gem mineral, corundum, is mined from assemblages rich in garnet, spinel, topaz, tourmaline and zircon occurring as residual, eluvial or alluvial deposits. Mineralogical studies of the rocks of the drainage basins of the gemming terrains reveal that gemstones are derived from garnetiferous gneisses, granulites and charnockites ‒ the latter being exceptionally rich in corundum, hypersthene and sillimanite when poor in quartz. Thus a desilication process under high temperature-pressure conditions is attributed to the formation of Sri Lanka's principal gemstones