Gems of Sri Lanka

The recovery of gems in Sri Lanka has over 2,000 years of history. Sri Lanka has long been renowned for varieties of corundum, chrysoberyl, spinel, garnets, beryl, tourmaline and zircons. With a land area of 65, 600 km2, Sri Lanka is the largest producer of various types of gem varieties per square kilometre in the world. Gems originate in the Highland Complex, and are also deposited mainly in the Highland Complex. Nearly 20% of the total land area of Sri Lanka is potentially gem bearing. In the 1970s, the gem industry became one of the major mineral-based, foreign-exchange earners of the country. The gem trade accounts for nearly 60% of the five mineral-based, foreign-exchange earning industries of Sri Lanka. Gem mining is a major occupation among landowners and gem industry, although in some areas mechanical mining is used. Finding a new gem-bearing location is largely a matter of chance, and this has led to much haphazard digging by traditional gem miners.

Gemstones occur mainly in paleoalluvial formations buried at some depth in the wide strike valleys and sometimes as in situ, residual, elluvial and alluvial deposits. Many tributary streams that carried down gemstones released during weathering of the bedrock of hills and their slopes deposited them in the lower valleys. The gem-bearing gravel layer lies at the base of the alluvium on or near the bedrock surface, which is generally 1–10 m, but sometimes even 30 m below ground level (Rupasinghe and Cooray, 1993). According to Dissanayake and Rupasinghe (1995), the gem deposits of Sri Lanka are classified into three major groups: sedimentary, metamorphic, and magmatic deposits. The sedimentary gem deposits are further subdivided into alluvial, elluvial, and residual, while metamorphic deposits consist of skarns and aluminous metasedimentary rock types. According to that classification, Kumbukkana-Okkampitiya gem fields consist of sedimentary and metamorphic types. The field observations indicate most of the deposits in the study area are residual and elluvial, except for those at Kumbukkan Oya and its flood plain, which appear to be residual and in situ types. Some elluvial type deposits, however, can also be found here. The gems found in the river sediments of the Kumbukkan Oya are elluvial-type gems rather than alluvial-type, rounded gems.

The gem-bearing gravel layer in the dry zone is found near the surface (Figure 14) or a few metres below ground (Figure 15). The dry climate helps in preventing the collapse of the walls of gem pits, and open pit mining is the common practice. The traditional open pit miners first remove the surface soil and extract the gem-bearing layer (Figure 15). The air-dried gem bearing gravel is sieved using a large sieve (2 mm) to remove the finer materials. Then the sieved material is taken to a water body and washed. (This method is rather different from the Ratnapura gem fields, where water is readily available. The wet zone of Sri Lanka has more than 2000 mm of rain per year.) Water may be pumped from the river into a pit, or water pumped from the mine is stored in a pit and then used to wash the gem-bearing gravel using the pan. In Ratnapura, there are many workers in a gem pit because deep mining is very complicated. There, gem mining is done as an occupation, which is payable weekly with various types of profit distributions according to the invested capital and participation of the members such as workers, work in chargers, and owners. Major investment is required for a mine in the Ratnapura area.

In the dry zone, mostly traditional gem mining is done on an individual basis or with only a few people. Here, gem mining is not a paid occupation; it is seasonal and considered as a supplementary income. In the dry season when they cannot cultivate crops due to shortage of water, the people mine gem pits. Although this mining is time consuming and hard work, it is relatively environmentally friendly. Almost all the gem pits are shallow, and they can be worked individually without high investment. Traditional mining in the Kumbukkana-Okkampitiya gem field is, therefore, economic. Riverbed mining is common to both Ratnapura and Kumbukkana-Okkampitiya gem fields.

The low water levels of the dry-zone gem fields help to minimize environmental and mining damage as compared to mining in the wet zone. Recently the banks of Kumbukkan Oya have been divided into blocks of about 100 to 150 m2 , and auctioned for large-scale mechanical mining—comparatively large scale for Sri Lanka. Heavy machinery such as caterpillars, bulldozers and tractors are used (Figure 16) to extract the riverbank soil, and very large craters have formed (Figure 17). The gem-bearing gravel layer in these craters is taken and washed in the river. Pumps are used to drain sediment particles from the bottom of the mine. As well, landowners adjacent to Kumbukkan Oya have recently begun mechanical mining on their own lands.

Table 2. Gem varieties found in Sri Lanka and their colours

Geology of Sri Lanka

Economic geology of Sri Lanka

Mining and the environment