SPECIFIC GRAVITY                             51 it is remembered that so far science has been unable (except in very rare instances of no importance) to manu­facture any stone of the same colour as the genuine and at the same time of the same specific gravity. Either the colour and characteristics suffer in obtaining the required weight or density, or if the colour and other properties of an artificial stone are made closely to resemble the real, then the specific gravity is so greatly different, either more or less, as at once to stamp the jewel as false. In the very few exceptions where chemically-made gems even approach the real in hardness, colour, specific gravity. &c, they cost so much to obtain and the diffi­culties of production are so great that they become mere chemical curiosities, far more costly than the real gems. Further, they are so much subject to chemical action, and are so susceptible to their surroundings, that their purity and stability cannot be maintained for long even if kept airtight; consequently these ultra-perfect " imitations " are of no commercial value whatever as jewels, even though they may successfully withstand two or three tests.
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