IMITATION STONES
The beryl glass mentioned in the previous chapter marks the transition
stage between manufactured stones which in all essential characters
are identical with those found in nature, and artificial stones which
resemble the corresponding natural stone in outward appearance only. In
a sense both sorts may be styled artificial, but it would be misleading
to confound them under the same appellation.
Common paste,[6] which is met with in drapery goods and cheap ornaments
in general—hat-pins, buckles, and so forth—is composed of ordinary
crown-glass or flint-glass, the refractive indices being about 1·53 and
1·63 respectively. The finest quality, which is used for imitations of
brilliants, is called ‘strass.’ It is a dense lead flint-glass of high
refraction and strong colour-dispersion, consisting of 38·2 per cent.
of silica, 53·3 red lead (oxide of lead), and 7·8 potassium carbonate,
with small quantities of soda, alumina, and other substances. How
admirable these imitations may be, a study of the windows of a shop
devoted to such things will show. Unfortunately the addition of
lead, which is necessary for imparting the requisite refraction and
‘fire’ to the strass, renders the stones exceedingly soft. All glass
yields to the file, but strass stones are scratched even by ordinary
window-glass. If worn in such a way that they are rubbed, they
speedily lose the brilliance of their polish, and, moreover, they are
susceptible to attack by the sulphurous fumes present in the smoky air
of towns, and turn after a time a dirty brown in hue. When coloured
stones are to be imitated, small quantities of a suitable metallic
oxide are fused with the glass; cobalt gives rise to a royal-blue tint,
chromium a ruby red, and manganese a violet. Common paste is not highly
refractive enough to give satisfactory results when cut as a brilliant,
and the bases are therefore often coated with quicksilver, or, in the
case of old jewellery, covered with foil in the setting, in order to
secure more complete reflection from the interior. The fashioning of
these imitation stones is easy and cheap. Being moulded, they do not
require cutting, and the polishing of the facets thus formed is soon
done on account of the softness of the stones.
A test with a file readily differentiates paste stones from the natural
stones they pretend to be. Being necessarily singly refractive, they
are, of course, lacking in dichroism, and their refractivity seldom
accords even approximately with that of the corresponding natural stone.
In order to meet the test for hardness the doublet was devised. Such
a stone is composed of two parts—the crown consisting of colourless
quartz or other inexpensive real and hard stone, and the base being
made up of coloured glass. When the imitation, say of a sapphire, is
intended to be more exact, the crown is made of a real sapphire, but
one deficient in colour, the requisite tint being obtained from the
paste forming the under part of the doublet. In case the base should
also be tested for hardness the triplet has been devised. In this the
base is made of a real stone also, and the coloured paste is confined
to the girdle section, where it is hidden by the setting. Sapphires and
emeralds of indifferent colour are sometimes slit across the girdle;
the interior surfaces are polished, and colouring matter is introduced
with the cement, generally Canada balsam, which is used to re-unite the
two portions of the stone together. All such imitations may be detected
by placing the stone in oil, when the surfaces separating the portions
of the composite stone will be visible, or the binding cement may be
dissolved by immersing the stone, if unmounted, in boiling water, or in
alcohol or chloroform, when the stone will fall to pieces.
The glass imitations of pearls, which have become very common in recent
years, may, apart from their inferior iridescence, be detected by
their greater hardness, or by the apparent doubling of, say, a spot of
ink placed on the surface, owing to reflection from the inner surface
of the glass shell. They are made of small hollow spheres formed by
blowing. Next to the glass comes a lining of parchment size, and next
the under lining, which is the most important part of the imitation,
consisting of a preparation of fish scales called _Essence d’Orient_,
When the lining is dry, the globe is filled with hot wax to impart the
necessary solidity. In cheap imitations the glass balls are not lined
at all, but merely heated with hydrochloric acid to give an iridescence
to the surface; sometimes they are coated with wax, which can be
scraped off with a knife.
PART II—SECTION A
PRECIOUS STONES