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Sri Lanka has been known for over 2,500 years for gemstones unique in quality and variety, and today they are the country's most renowned and important mineral resource. Gemstone deposits lie within a geologically narrow zone. Recent geological surveys suggest that the potential for gemstones is about 50 percent higher than hitherto expected and many new gem fields have been located in the central highlands and southwestern geological formations. Gem minerals are obtained from eluvial, alluvial and residual formations.

The best known and most prolific gem producing area in the Island is the Ratnapura district of the Sabaragamuwa province, the most extensively gemmed areas being the neighborhood  of Eheliyagoda, Kuruwita, Ratnapura, Pelmadulla, Balangoda, Kalawana and Rakwana. The Sabaragamuwa catchment drained by the Kalu ganga, the major river in the area, has an area of approximately 4 500 square kilometers. It is also of interest that this region is an area of heavy rainfall, and is subject to serious flooding during the wet seasons. Smaller extensions of the Sabaragamuwa gem fields are found in the Western and Southern provinces. 

All these areas are the traditional gem areas where gem mining has been going on for centuries. But mining activity in the past three decades or so in the upper reaches of the Menik ganga in the neighborhood of Buttala and Okkampitiya in the Uva province, and still more recently in the Elahera area in the Matale district of the Central Province, indicate that the gem bearing areas are not confined to the south-west sector of the Island . Further prospecting operations in these new regions are likely to, reveal gem bearing gravels though they are unlikely to be as extensive or as prolific as the Sabaragamuwa gravels. Other locations which have produced some stones in the past are the  Nuwara Eliya basin, the Horton Plains, Maskeliya and the Kandy district. With the exception of moonstones, which are obtained from  partly altered pegmatites, near Ambalangoda, all other gem  material is obtained from alluvial deposits of rivers and streams, or in abandoned channels, draining regions containing gem bearing  crystalline rocks.

 Gemstone Deposit Formation And Their Characteristics

mapThe gem material  is thus of secondary  origin, occurring as worn fragments or rolled pebbles in gravel beds inter-layered with beds of sands and clays. The gem material in the gravel beds represent the chemically inert material brought down during past geological ages by the action of streams and rivers and deposited in the low lying     valleys and  swamps   and  preserved during transport  and the rolling action of water due to their superior hardness. Other minerals of non-gem variety which possess  similar characteristics are  associated with the gem material  in the  gravels; examples of these  are  ilmenite, thorianite, baddeleyite,  geikielite and others.The  gravels are thus formed  by the action of running water in streams and rivers and laid   down in their  beds  or in  their  flood  plains.

During times of heavy floods, the carrying power of the streams is increased several fold and  the load of heavy material is carried further away, whereas in the dry seasons the carrying  power is limited and deposition  takes  place much  nearer the source of the material. Sudden drop in velocity  for any reason has the same effect and results in deposition; this  commonly happens when a mountain stream falls into a slower moving main stream. Further, deposits laid down in one geological age  may be eroded and  deposited elsewhere in another geological age. As a consequence of the capricious behavior  of streams and  rivers, the gem bearing gravels do not have any  great lateral extensions. Rather they tend to occur as  pockets, lenses and streaks in layers of sands and clays with much barren ground in between and the gravels are strictly local in their distribution.

The depth, character and composition of the gem gravels vary considerably from place to place and even in a particular location, but one thing they all have in common is the presence of rounded pebbles of ubiquitous quartz varying from half inch to several inches. The overburden capping the gravels consists of layers of sands, sandy clays, ferruginous sands, clays and peaty clays. The thickness of the gravel beds also shows much variation from a few centimeters to a meter and more. Their depth also varies, from shallow depths less than three meters to twenty meters and more. The gem bearing layer is known as the "illam" and it generally rests on the decomposed rock which is normally highly kaolinised. The altered rock base is referred to as the "malawa" by the gem miners. In several locations, more than one gravel layer is present and where this is so, the lowest layer is the most prolific in gems. The pits are stopped when the altered rock is encountered.