Naturalis historia 37.200: Gem-bearing rivers (Acesines, Ganges); India the land of gems

37.200

decussi fragmenti, quod in lamina ferrea uratur, efficacissimum experimentum excusate mangones gemmarum recusant, similiter et limae probationem. obsianae fragmenta veras gemmas non scariphant, in ficticiis scariphatio omnis candicat. iam tanta differentia est, ut aliae ferro scalpi non possint, aliae non nisi retuso, omnes autem adamante. plurimum vero in iis terebrarum proficit fervor.
Gemmiferi amnes sunt Acesinus et Ganges, terrarum autem omnium maxime India.

CHAP. 76. (13.)—THE METHODS OF TESTING PRECIOUS STONES. On the contrary, we will make it our business to point out the methods of detecting these false stones, seeing that it is only proper to put luxury even on its guard against fraud. In addition to the particulars which we have already given, when treating of each individual kind of precious stone, it is generally agreed that transparent stones should be tested by a morning light, or even, if necessary, so late as the fourth hour, but never after that hour. The modes of testing stones are numerous: first, by their weight, the genuine stone being the heavier of the two; next, by their comparative coolness, the genuine stone being cooler than the other to the mouth; and, next to that, by their substance; there being blisters perceptible in the body of the fictitious stone, as well as a certain roughness on the surface; filaments, too, an unequal brilliancy, and a brightness that falls short before it reaches the eye. The best mode of testing is to strike off a fragment with an iron saw; but this is a thing not allowed by the dealers, who equally refuse to let their gems be tested by the file. Dust of Obsian stone will not leave a mark upon the surface of a genuine stone: but where the gem is artificial, every mark that is made will leave a white scratch upon it. In addition to this, there is such a vast diversity in their degrees of hardness, that some stones do not admit of being engraved with iron, and others can only be cut with a graver blunted at the edge. In all cases, however, precious stones may be cut and polished by the aid of adamas; an operation which may be considerably expedited by heating the graver. The rivers which produce precious stones, are the Acesinus and the Ganges; and, of all countries, India is the most prolific of them.