Within the earth, there are also natural processes going on which affect rocks. Continuous lateral pressure may cause an interruption in the strata, and fine, flat particles may be induced to re-arrange themselves. Slipping may result, and faults appear in the strata. Twisting, and up and down movements cause other fractures or weaknesses, known as joints. One need not go far to see such examples in our own districts.
As for the majority of gem stones, these are found in the earth’s crust where they were formed, or in secondary deposits to which they have been carried by natural agencies, often some distances away from the original place of formation. In the first instance, they are often a constituent of the rock itself, being formed at the same time as the other component minerals. Sometimes they are attached to walls or cavities in the rock, and in these instances they are not completely embedded. These cavities, known as “druses,” are of a more recent date than the parent rock, and the mineral lining the cavity has usually assumed a definite shape in its formation, that is, it appears as a crystal or mass of crystals. Only one end of the crystal is attached to the parent rock. But when discovered by the miner, these crystals are often found to be broken, or irregular in shape, on account of disturbed conditions. Amethyst is an example of a stone often found in druses, while peridot is an example of a gem stone which has formed at the same time as the surrounding minerals.
But a large number of gem stones are found in secondary deposits, especially in so called gem gravels. These may be in river beds, or former river beds. A variety of stones are found together, and the gravel pits of Ceylon may be cited as a good example where a number of different gem stones may be found associated with other minerals. In such instances, deep mining is not necessary since the gems are found near the earth’s surface.
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