ost magnificent in fire, beauty and
purity ever discovered.

The positions and angles of the facets, as well as the number, are of
supreme importance, and diamond cutters--even though they have rules
regulating these matters, according to the weight and size of the
stone--must exercise the greatest care and exactitude, for their
decision once made is practically unalterable.

CHAPTER XII.

IMITATIONS, AND SOME OF THE TESTS, OF PRECIOUS STONES.

We now arrive at the point where it is necessary to discuss the
manufacture and re-formation of precious stones, and also to consider a
few of the tests which may be applied to _all_ stones. These are given
here in order to save needless repetition; the tests which are specially
applicable to individual stones will more properly be found under the
description of the stone referred to, so that the present chapter will
be devoted chiefly to generalities.

With regard to diamonds, the manufacture of these has not as yet been
very successful. As will be seen on reference to Chapter II., on "the
Origin of Precious Stones," it is generally admitted that these
beautiful and valuable minerals are caused by chemically-charged water
and occasionally, though not always, high temperature, but invariably
beautified and brought to the condition in which they are obtained by
the action of weight and pressure, extending unbroken through perhaps
ages of time.

In these circumstances, science, though able to give chemical
properties and pressure, cannot, of course, maintain these continuously
for "ages," therefore the chemist must manufacture the jewels in such
manner that he may soon see the results of his labours, and though real
diamonds may be made, and with comparative ease, from boron in the
amorphous or pure state along with aluminium, fused in a crucible at a
high temperature, these diamonds are but microscopic, nor can a number
of them be fused, or in any other way converted into a large single
stone, so that imitation stones, to be of any service must be made of a
good clear glass. The glass for this purpose is usually composed of
53.70 per cent. of red lead, 38.48 per cent. of pure quartz in fine
powder, preferably water-ground, and 7.82 per cent. of carbonate of
potash, the whole coloured when necessary with metallic oxides of a
similar nature to the constituents of the natural stones imitated. But
for colourless diamonds, the glass requires no such addition to tint it.