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. Other gems are of magmatic origin.
Residual deposits are mainly found in flood plains of rivers and streams. The metamorphic types of gems constitute 90% of the gem deposits in Sri Lanka. Sri Lanka has the highest density of gem deposits compared to its landmass. Ratnapura contains the most gem deposits and derived its name from the gem industry. Ratnapura means “city of gems”.
The blue sapphires from Sri Lanka are known as Ceylon Sapphire. Ceylon Sapphires are reportedly unique in colour, clarity and lustre compared to the blue sapphires from other countries.