Gem corundum crystals, on account of their great hardness and freedom from weathering, have survived the wear and tear of the years they should be watched for in panning for gold. In Burmah, long the producer of the finest rubies, they are found in crystalline limestone and gneiss, but are mostly obtained from placers formed by the weathering of these rocks, Sapphires are found in a peculiar lamprophyre in Montana, and also in andesite in the same state. In North Carolina, these gems have been found in chlorite, mica, and talc, in a dike of olivine and serpentine. In New York and New Jersey, rubies have been found in crystalline limestone that is banded with gneiss. They may be found in syenite, nepheline-syenite, anorthosite, peridotite, serpentine, and other basic igneous rocks, particularly when these rocks are pegmatite. Rubies and sapphires are corundum crystals of red and blue color, respectively. Diamond scratches quartz easily quartz scratches glass therefore the ability to scratch glass is not a test for diamond. They can be recognized by their rounded form, the curved faces of the crystals, and their extreme hardness. In panning for gold, diamonds should be watched for.
As diamonds of considerable size have been found in the glacial drift of some of the Northern States, there is a chance of finding a diamond field in Ontario, from which much of the drift was derived. Diamonds of merchantable size and in considerable quantity have been taken out of weathered peridotite near Murfreesboro, Arkansas. Diamond placers occur in South Africa, India, and Brazil and small diamonds have been found in California placers. In the productive diamond fields of South Africa, the gems are found in weathered rock in volcanic necks. As peridotite and serpentine are plentiful in the Precambrian of Canada, diamonds should be looked for in these rocks. In Brazil, they are found associated with a peculiar flexible sandstone, itacolumite. Diamondsĭiamonds are found in peridotite and serpentine, and in the soft material formed by the weathering of these rocks. One ounce Troy equals about 151¾ carats. It should always be remembered that crystals of unusual beauty and perfection may have considerable value as mineral specimens. A carat is 205 milligrams, or 3 1/6 grains, very nearly. Vugs and rounded masses that may be vugs should be carefully examined for gem crystals. The weight of gems is given in carats.
Pegmatite dikes are therefore apt to yield gems. Most gems are crystals of unusual quality in regard to transparency and color they are to be looked for in those rocks that have crystallized slowly, or in cavities that have been slowly filled by the growth of crystals.
This interesting list may lead to a more careful watch for precious and semi-precious stones and their names.
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