materials
travelling far, and heavier material being left behind in pockets or
holes and covered up by mud, sand or gravel. Most gem-stones occur in
such alluvial deposits, their occurrence being governed by the presence
of gem-bearing rocks in the neighbourhood.
The
gem-stones recovered from gravels are worn to some extent—the edges are
rounded, faces dulled and the crystals changed to rounded pebbles.
Flaws and cracked portions may be removed during the process thus
yielding the best gem material, as in the case of the diamonds of India
and the rubies and sapphires of Burma and Ceylon. The gravels ai;e
panned for gold, which lead to the dis- ' coveries of gem minerals.
These rounded pebbles of gem minerals are generally not easy to
recognize without some knowledge of gemmology.
Most
of the valuable gems occur in alluvial deposits in deep valleys of
mountainous countries. Mining is usually carried on by people in small
groups according to primitive methods. A prospecting pan often serves
their purpose. These gem washers have great skill in washing the gravel
and separating gem-stones. But the more advanced methods of the present
day consist in the use of a jig or jigger, which is also used to
separate metallic ores from their gangue. In large plants, as in the
Kimberley diamond mines, the material is passed to the jigs, after
washing in circular pans. These methods require a large amount of
water. In dry areas, where water is scarce, winnowing or blowing by the
wind is practised. This carries away the dust and lighter material and
the rest is hand-sorted.
Workings
in alluvial deposits are small, but the Burma Ruby Mines Co. used more
efficient methods, and systematiÂcally dealt with large amounts of
material. The Burmese wash down the material on the hillside with a
powerful jet of water or hydraulic mining (hydraulicking). In the Vaal
river in Africa, the whole- gravel is excavated from the river bed
after diverting its course.
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