A recipe from ancient mountains
The gem-bearing gravels of Ratnapura are secondary alluvial deposits, a concentrated slurry of resilient minerals left behind after millions of years of erosion. The story begins in the Precambrian rocks of Sri Lanka's central Highland Complex, which makes up 90% of the island. These rocks, formed between 550 and 2,400 million years ago, were subjected to intense heat and pressure during the formation of the Gondwana supercontinent. This granulite-grade metamorphism created the perfect conditions for gem formation within source rocks like garnetiferous gneisses, charnockites, skarns, and marbles.
Over geologic time, weathering and monsoonal rains wore down these mountains. Rivers like the Kalu Ganga transported the eroded material, carrying a mix of minerals from these varied parent rocks. Because gemstones like corundum (sapphire and ruby), spinel, and chrysoberyl are extremely hard and dense, they survived while softer minerals disintegrated. The rivers deposited these heavy minerals in flood plains and ancient stream channels, creating a concentrated layer of gem-bearing gravel known locally as "illam." This process makes Ratnapura a collection point for minerals from a vast and diverse geological terrain. The illam itself is found at depths ranging from three to twelve meters, and sometimes much deeper.