Mineral Guide > Gems > Mining of Gems

Mining of Gems

The methods employed in the mining of gems depend obviously upon the occurrence of the latter. If occurring in gravels, or decomposed rock areas, as is the case with the majority of gems, mining usually takes the form of open cuts, made either by digging numerous small pits, or one of extensive dimensions. The separation of the gems from the common pebbles accompanying them is then performed by some method of washing, usually hand panning combined with hand picking. Panning depends for its operation upon the generally high specific gravity of the gem minerals as compared with those of commoner occurrence, and is thus similar in principle to gold panning. The utensil most commonly employed is a shallow pan of wood or metal, from 12 to 18 inches in diameter, and of a more or less conical shape. On taking up a quantity of the gem-bearing gravel in this with water, and rinsing the whole with a circular motion, the lighter minerals fly off and the heavier concentrate toward the center. After the contents of the pan have in this manner been considerably reduced, by searching and hand picking any gems which may have gathered at the center can usually be readily seen and picked out.

Of the methods of mining and separating gravels by hand digging and panning, the procedure of the Cingalese in exploiting the Ceylon gem gravels may be considered a good illustration. These methods are thus described by Dr. A. C. Hamlin:
"The mining operations are generally carried on by the native Cingalese, who labor in the light of a pastime, and only during intervals of their agricultural employments. Some few, however, undertake the labor as a regular business, but they belong to a low and dissipated class, and do not work systematically or with regularity. Therefore, the gem-mining of Ceylon cannot be regarded as a fixed and permanent business.

When an exploration has been determined upon, a small party of villagers set out for the promising region provided with the implements of mining and the means of camping out. The times selected for the operations are after the heavy rains, which pervail in June and October, and the floods have subsided. The beds of rivers, or smaller streams, are often chosen as easier of access than the plains. If the river-bed is selected, the first act of the explorers is to seek for the proper locality where the gem-bearing strata may be found. To ascertain this, the Cingalese thrust along iron rod of ten or twelve feet in length into the earth, and test the nature of the sub-soil. By means of long practice the natives can adroitly penetrate the earth to a considerable depth; and, by the resistance to the movement of the rod, can detect the gem deposit of which they are in search.

"If the indications are good, the natives proceed to build a hut if they are at a distance from their village, and prepare for the operations, which often extend over many weeks. After diverting a part of the force of the stream so as to form a quiet pool, they proceed to excavate the sand and gravel within a certain area. In order to accomplish this they use hoes with handles fifteen or more feet in length. The top strata are hurriedly raked up and thrown away; but as the pit deepens and the gem stratum is approached, the work is performed with greater care. As soon as the hoes bring up fragments and boulders of white quartz, or strike thin ferruginous crust, every particle of the gravel drawn up is carefully preserved. The gravel and sand thus obtained are then placed in large baskets woven of split bamboo and shaped to a conical point at the bottom. The basket thus filled is placed in the current of water, and its contents washed by imparting to it a circular motion. This washing process is kept up until the stones, gravel, and lesser particles are cleansed. During this operation the gems, which are much heavier than common stones, settle at the bottom of the basket, and are there collected together, so that when the superincumbent gravel is removed the sapphires, garnets, zircons, etc., are easily discovered at the bottom and removed. This is the manner in which the wet diggings are carried on, and is the easiest mode of exploration; but it is by no means as sure, or often as profitable, as the operations in dry ground on the river banks or in the plains. The dry diggings are much more laborious, as the soil is firmer, and the gem strata must be transported to water to be washed and sifted. These dry deposits are found the richest beneath the alluvial plains, which seem to have been in distant times shallow lakes and lagoons.

" The gem stratum, called mellan, is always well defined, and occurs at a certain depth, which seems to correspond to the bottom of the lake at a definite period. This depth varies from two to twenty feet, and is perhaps even greater; but the natives rarely excavate below the depth of twenty feet. This peculiar formation, which is generally 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."

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 of 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 earthly 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 surface openings and cuts, some of the latter very extensive. Water is carried from Yogo Creek, ten miles distant, by a ditch and flume, with a parallel hydraulic pipe line ; and a system of sluices extends all along the company's workings.

"Where the rock is much decomposed, the hydraulic process is employed largely; as it becomes harder, power is necessary to break it up. Then the rock is thrown out in dumps and allowed to disintegrate by exposure to the weather, as with the African "hard blue." This process requires from a month to a year, according to the condition of the material. Sometimes a stream of water is turned on the dumped rock, and the process thus expedited. When sufficiently decomposed, this material is subjected to the same washing process as the material naturally disintegrated.

" In the washing the fine earth is carried away with the water; all hard lumps remaining are again thrown out on a dump to decompose further; and the sapphires, after several screenings, are picked out by hand."

An interesting discovery made in South Africa, in connection with the process of sorting diamonds by concentrating them on percussion tables, was that if the tables were covered with thick grease the diamonds, and even other precious stones, such as rubies, sapphires, and emeralds, would adhere to the grease and be held, while the valueless ingredients of the rock would pass by. The grease can be used for this purpose for only a few hours, when it must be scraped off and a new coat applied. This, however, is a small disadvantage compared with the great gain afforded by the selective power of the " greaser," as it is called.

Mining for gems by methods of tunneling, shafts, and other means employed in deep mine workings is rarely carried on. In the first place, gems do not often occur in definite veins as do the precious metals, being more commonly irregularly distributed in pockets through the rock. In the second place, little really systematic mining of gems is carried on. As a rule, the occupation is, or has been, a rather desultory one. A find of a few good stones leads to temporary search and exploration, lasting for a few years perhaps, then the work proves no longer profitable and is abandoned until new finds arouse new hope and revive the industry.

The element of fortune, good and bad, seems to prevail more largely in the mining of gems than in even that of the precious metals. In gem-mining, as in that for gold and silver, great labor and little reward go side by side with little labor and great reward. Moreover, the distribution of gems is exceedingly irregular, and their market price varies within wide margins, from circumstances of fashion, supply, general financial conditions, etc.

Yet these contingencies might doubtless be largely overcome by intelligent and broad-minded management, such as has been conspicuously displayed in the conduct of the diamond mines of South Africa. Not only is the mining here conducted according to the most approved systems of modern engineering, but equal attention is paid to placing the gems upon the market, so that an over-supply shall not reduce the price.

Regarding the influence of increase of depth upon the distribution and quality of the gem minerals, no principles have been established as yet. It is known that veins of amethysts, for instance, have turned entirely colorless on penetrating below the surface, so that a valuable stone became with depth worthless. On the other hand, improvement in color and quality of stones below the surface, as compared with those above, may often be reasonably expected, since the latter are more exposed to disintegration and weathering, and the fading effects of light.

In the mining of gems in a small way the amateur is likely to make the mistake of resorting to the use of too much powder or other explosive. While the rough work of exploration may wisely be carried on by means of blasting, the actual removal of the mineral from the matrix should usually be performed, where possible, by picks and chisels, in order to avoid the shattering and breaking of pieces suitable for gems, which often happens in blasting. Many fine gems have been lost through carelessness in the work of mining, and while not all losses of this kind can be avoided, with care and patience they can be reduced to a minimum.