One of the popular pegmatite gem deposits in Sri Lanka is the moonstone mine in an area named Meetiyagoda, off the coastal town Ambalangoda in the southern part of the island. A pegmatite vein had cut across the metamorphic rock in this area. The whole area is a flat land, with paddy fields, marshy lands and ponds like area, some of which may probably be the abandoned places of mining.
Several mines were in existence extracting high grade moonstones from the feldspar mineral, but today there are two mines in normal activity. Mining is carried by sinking vertical shafts similar the deep mining method adopted in other areas in Sri Lanka for deep mining of alluvial deposits. In these mines the pits are reinforced with coconut timber cross beams, and fern foliage is pushed between the sides of the pit and vertical supporting struts from areca-nut trees which are driven behind the cross-beams, to prevent the damp side wall from caving-in.
Since the area is low-lying marshy, water has to be pumped out regularly and this is done using fuel operating water pumps. Broken rough material is brought to the surface in gunny bags or cane baskets using winches. Material called illama in local terms is collected at one place for washing. A specially constructed pond close the mine has been used for washing of the rough. Large cane baskets are used for washing to clean the hard gem material from clay. After cleaning the material from clay, under the direct sun shine, gem quality moonstone rough pieces are handpicked, from other solid material which main consists of quartz.
This pegmatite vein contains 50% clay, 40% feldspar, 5 % quartz, smoky quartz, opaline silica and traces of sulphides and tourmaline.
Fig. 01Rough moonstone containing inter-bedded large quartz crystal pieces
Moonstone gem material found in this area is of the orthoclase end of the feldspar group. This variety of moonstone is colourless to white body colour with a white to fine blue shiller effect called adularescence. This special optical effect is produced due to the inter-grown layers of albite mineral in fine form in orthoclase. Orthoclase is a potassium aluminium silicate KAlSi3O8 and albite is sodium aluminium silicate NaAlSi3O8, both are of the end members of the isomorphous series of Alkali-feldspars. Orthoclase has a monoclinic crystal structure and albite with a triclinic structure.
Clay material is the decayed form of feldspar, which is collected from these moonstone mines for production of ceramic materials. Gem quality rough moonstone material is removed from partly decayed feldspar rock which is inter-grown with quartz and other minerals. Large crystals of quartz of colourless and smoky varieties are also found in this pegmatite vein.
Although large percentage of moonstone rough is collected after washing, only a small percentage is taken as clean good quality material without any visible inclusions for cutting and polishing. Fashioning of moonstone is done by majority of gem cutters in the traditional methods even today. Moonstone is a less hard material with hardness of 6 of the Mohs’ scale, and grinding and polishing is easy. Rough grinding and pre-forming of the materials is done on a circular caborandum plate fixed to wooden barrel which is rotated by a bow string. Polishing is done in the same type of machine with a polishing plate or on a wooden plank using locally prepared polishing medium.
Material is mostly cut usually on double cabochon, and also as beads. This material is usually mounted to silver jewellry
Traditional type of gemstones mine in Sri Lanka
Washing the rough to extract moonstone rough and other minerals from kaolin moonstone.
Packing the moonstone rough and kaolin separately into bags.
Meetiyagoda moonstone mine is a tourist spot of tourist attraction