Gem mining in solid rock is exceptional. Diamond mining of South Africa is an example. Pegmatite veins often contain gem minerals, and they have been worked for quartz, topaz and tourmaline in Burma and for tourmaline and beryl in Brazil and South-West Africa. At times these pegmatites are weathered to a soft material, which is easily worked, e.g. the sapphire-bearing pegmatites of Kashmir. The emerald mines of Colombia are open quarries in limestone.
The only instance, where gem mining is done by modern methods using large scale mechanical aids is provided by the diamond mines of South Africa. The 'Premier' mine near Pretoria is the largest open quarry on earth. The Kimberley mines were worked as open quarries to a depth of about 1,300 ft. after which underground mining had to be resorted to as in the case of coal or metal mines. The rock is broken up, carried to the surface, then crushed, washed, jigged and dealt with just like an ordinary ore. The' Blue ground' forms the diamond ore. In South-West Africa, the diamond is found in a sand dune along the coast. The dunes are attacked with mechanical excavators mounted on caterpillar trucks driven by electricity. They pick up 5-10 tons of material at each scoop. The material is carried to the works where it is sieved, panned or jigged. The final concentrate contains small diamonds.
Chapter III
CHEMICAL COMPOSITION OP GEM-STONES
GEM-STONES are well known from ancient times but a knowledge of chemistry is of modern growth. For this reason gem-stones are more known by their ancient names than by their composition. Chemists know of 92 elements, which may occur in nature singly or combined. Aluminium and oxygen are elements which when combined form
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