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 fourth1
hour, but never after that hour. The modes of testing2
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.
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 Obsian4
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;5
which may be considerably expedited by heating the graver.
The rivers which produce precious stones, are the Acesinus6
and the Ganges; and, of all countries, India is the most prolific