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Historical occurence of Gems in the World

Historical occurence of Gems in the WorldIt was the opinion of the ancients that precious gem stones and rocks were largely confined in their occurrence to tropical countries. Most gems which they knew were so obtained, India being the chief source of them. Their wise men reasoned, therefore, that the warmth and light of the sun of the tropical zone were needed to give gems that fire and brilliancy which made them precious among stones.

With the wider knowledge of the earth which has been gained in later times, however, it has become evident that climatic conditions have little or nothing to do with the occurrence of gems. The greater oxidation produced by the heat of the sun in the tropics may add to the warmth of color of such stones as the carnelian and agate, but it would have a tendency -to fade the amethyst and sapphire.

A greater abundance of gems in the tropics may arise from more extensive decomposition of the rocks there, and this is undoubtedly a favorable circumstance. Moreover, glaciated countries, such as the northeastern portion of North America, have a soil composed of too heterogeneous a mixture to favor the search for gems. So far as the underlying rock is concerned, however, there is not, so far as we know at present, any distribution by latitudes which favors one locality over another. Hence the mountain fastnesses of the Urals furnish gems no less than the broad valleys of India, the bleak shores of Labrador as well as the steaming jungles of Burmah, and the barren veldt of the Transvaal as well as the thickly settled valleys of Bohemia.

The first discovery of gems in a region is usually made, like that of gold, in the beds of streams. Often it is in the search for gold that gems are found, as is illustrated by the fact that the discoveries of diamonds in Brazil, sapphires in Montana, and rubies in North Carolina were made in this manner.

The frequent occurrence of gems in the beds of streams is due to the fact that the gem minerals are usually harder and less easily decomposed than the other minerals of the rocks in which they were formed. Hence they remain after the mother rock has disintegrated and its constituents for the most part removed. The discovery of gems in a stream bed is further facilitated by the enhancing of their color when wet, 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 Fauresmith, 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 action and currents the cliffs of the shore are continually being worn down, and the lighter and finer particles borne sea-ward, while those which are heavier, either because of higher specific gravity, or of greater resistance to erosion and decomposition, and hence larger, remain behind. A continual concentration is thus going on which in time may produce gem deposits of some extent.

The area upon which such a deposition may take place is, however, relatively narrow at any one period, as compared with that afforded by streams, and hence few gems are likely to be obtained from such sources. Labradorite and hypersthene are obtained from deposits of this character upon the coast of Labrador; chlorastrolite from the shore of Isle Royale; and agate and thomsonite from beaches of Lake Superior.

Hardly any other gem minerals can be mentioned as so obtained, with the exception of amber, which is gathered from the coast of the Baltic Sea. This, however, is deposited not through its heaviness but its lightness, it being borne upon the waves and tossed inland.
Passing from the gravels in which gems are found to a consideration of their original rock matrices, it may be said that rocks of the kind known as metamorphic are more commonly than any others the home of the gem minerals.

Meta-morphic rocks are those which have been changed by heat and pressure, or chemical agencies, from their original condition. They include crystalline limestones, quartzites, mica and hornblende schists, gneisses, eclogites, etc. The rubies of Burmah, the emeralds of the Urals, the diamonds of Brazil and the garnets of the Alps are illustrations of gems which occur in this way.

-Next to metamorphic rocks those of an eruptive character afford the gem minerals in the greatest abundance. Of these the acidic rocks, i. e., those containing a relatively large quantity of silica, such as the granites, trachytes, rhyolites, and syenites, are the most prolific.

The coarsely crystallized form of granite known as pegmatite is especially fertile in the gem minerals. The basic eruptive rocks, i. e., those poor in silica, afford among gem minerals, chrysolite, some garnet, some corundum, vesuvianite, and a few others. They are, however, comparatively barren. The diamonds of South Africa occur in a rock seeming to be of a basic eruptive character ; but whether the diamonds are of primary or secondary origin is not yet known.

Of all the great groups of rocks those of sedimentary origin furnish the fewest gems. Those which do occur in these are for the most part probably derived from older eruptive rocks. Such is believed to be the origin of the emeralds of Colombia, which are found in a bituminous limestone of Cretaceous age. The opals of New South Wales, however, occurring in sandstones and limestones of Tertiary age, doubtless were formed in place, and owe their deposition to the circulation of siliceous waters through the rocks.

The distribution of gems through a rock or gravel matrix is not usually uniform. The gems more commonly occur in pockets, so-called, the location of which seems to be governed by no law as yet discovered. Where crystallization of minerals has taken place about a fissure or open cavity, the minerals are more likely to be clear and free from inclusions than where formed in the mass of rock itself.