causing
them to attract attention. Moreover, the flowing stream tends to group
together minerals of the same specific gravity, thus causing a
concentration of the gem minerals. A stream bed is therefore a good
place to look for gems. Besides the fact that the gems are concentrated
here, and can more easily be seen, a further advantage lies in the fact
that they are likely to be of better quality than those found in the
matrix, since the wear of the stream has opened and separated them
along any little seams that may have existed, and the pieces left will
be of uniform texture and free from imperfections. On the other hand, a
continual reduction in size takes place from the wear of the stream,
and larger gems will therefore be obtained by searching the mother
rock. The quantity of any given gem is likely, too, to be limited in a
stream deposit as compared with the deposit in place; and just as with
gold, the mother lode must eventually be sought if a permanent supply
is desired.
It
must not be supposed, however, in speaking of stream-bed deposits, or "
gravels," as they are usually called, that only gravels over which
water is now flowing are meant. Beds of earlier streams will afford the
same products and the same facilities, with the exception that the
color of the precious stones will not be so obvious. It is evident,
too, that in any particular gravel the quantity, size, and variety of
the gem minerals present will depend not only on their quantity and
variety in the original rock mass of which they formed a part, but on
the length of time they have been exposed to wear and the rate of flow
of the stream.
In
the so-called gem gravels, numbers of gem minerals are usually
associated together. Thus, in those of Ceylon are to be found sapphire,
tourmaline, zircon, garnet, spinel, iolite, and many others; -and in
those of Brazil, topaz, chrysoberyl, andalusite, and others. Quartz,
garnet, and beryl are frequent constituents of gem gravels, as well as
the heavier minerals ilmenite, rutile, and magnetite. The knowledge
that garnets usually accompanied diamonds in the " wet diggings" along
the Vaal River led to the discovery of the " dry diggings " at
Faure-smith, in South Africa, and in other cases a knowledge of the
minerals usually associated with a gem has been of great aid in
discovering the gem itself. This grouping together of the gem minerals
arises from the fact that they are not only formed together in the
original rock mass, but also that they are of about the same hardness,
and to a certain extent, specific gravity.
The
beaches of lakes, or of the sea, also afford places for the gathering
of gems by processes similar to those just described. By wave
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