Sri Lanka, geologically speaking is an extremely old country. Ninety percent of the rocks of the island are of Precambrian age, 560 million to 2,400 million years ago. The gems form in sedimentary residual gem deposits, eluvial deposits, metamorphic deposits, skarn and calcium-rich rocks. Nearly all the gem formations in Sri Lanka are located in the central high-grade metamorphic terrain of the Highland Complex. The gem deposits are classified as sedimentary, metamorphic and magmatic; the sedimentary types being the most abundant. The mineralogy of the gem deposits varies widely with, among others, corundum (sapphire, ruby), chrysoberyl, beryl, spinel, topaz, zircon, tourmaline, garnet being common.