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horizontal,
is composed of a conglomerate of quartz gravel resting upon or mixed
with a stiff clay, often indurated by a ferruginous oxide. In among
this cascalho, or just below it and adhering to it, are found
the fine pebbles and crystals of sapphire, tourmaline, garnet, zircon,
spinel, and chrysoberyl. Under these rocks, and in peculiar hollows in
the plastic clay, which the natives call ' elephants' footsteps,' the
gems are found clustered together heterogeneously, and often so
perfect in form as to appear as though created there. At other places
they are collected together in these pockets, in such a manner as to
suggest the idea that they had been washed in by a current of water."
An
account of the methods of gem mining in Brazil, which in many respects
are similar to those above described, will be found in the chapter on
the Diamond in this work. Such methods may be considered typical of
the mining of gems on a small scale. Their success will obviously
largely depend upon the skill and care of the individual miner. In
countries where hand labor is cheap such methods can usually be
conducted with better profit than can be afforded by the use of
machinery. This will especially be true if the gem deposits are, as is
often the case, scattered over a wide area and are irregular in
quantity.
The
part of the operation of gem mining to which some form of machinery or
apparatus can usually be most profitably applied, is that of washing or
concentration.
The
machines employed for this purpose may vary from the crude "baby" of
the South African Vaal River miner to the elaborate jigs and pulsators
of the Kimberley mines.
Most of these methods are patterned after those of gold placer mining, and depend for their success upon the same principle.
The
mining of sapphires in Montana affords an illustration of a
combination of several methods of washing, which typifies what may be
done in this manner. It is thus described by Mr. George F. Kunz in the
Mineral Resources of the United States for 1901:
"The
methods employed are a curious combination of those of the California
gold-workings and the South African diamond mines. As in the latter,
the gangue of the gems is an igneous rock, hard below but decomposed
above, in varying degrees, to a mere earthy mass at the surface. From
this last the gems are separated by washing and sluicing, much in the
manner of placer gold; though, because of the less density of
sapphires, more care is necessary, and the sluice boxes must be less
inclined, to prevent the gems from being carried over the riffles. Most
of the New Mine Syndicate's workings are sur-
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