lly, they are
much cheaper than a natural stone of the same size and weight.

Some spurious stones have their colour improved by heat, by being tinged
on the outside, by being tinted throughout with a fixed colour and
placed in a clear setting; others, again, have a setting of a different
hue, so that the reflection of this shall give additional colour and
fire to the stone. For instance, glass diamonds are often set with the
whole of the portion below the girdle hidden, this part of the stone
being silvered like a mirror. Others are set open, being held at the
girdle only, the portion covered by the setting being silvered. Other
glass imitations, such as the opal, have a tolerably good representation
of the "fiery" opal given to them by the admixture, in the glass, of a
little oxide of tin, which makes it somewhat opalescent, and in the
setting is placed a backing of red, gold, copper, or fiery-coloured
tinsel, whilst the glass itself, at the back, is painted very thinly
with a paint composed of well washed and dried fish-scales, reduced to
an impalpable powder, mixed with a little pure, refined mastic, or other
colourless varnish. This gives a good imitation of phosphorescence, as
well as a slight pearliness, whilst the tinsel, seen through the paint
and the curious milkiness of the glass, gives good "fire."

A knowledge of the colours natural to precious stones and to jewels
generally is of great service in their rough classification for testing,
even though some stones are found in a variety of colours. An
alphabetical list of the most useful is here appended, together with
their average specific gravities and hardness. (See also Chapter VII. on
"Hardness," and Chapter VIII. on "Specific Gravity.")

WHITE OR COLOURLESS STONES.

_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)

Beryl 7-3/4 2.709-2.81
Corundum 9 3.90-4.16
Diamond 10 3.502-3.564
Jade 7 3.300-3.381
Opal 5-1/2-6-1/2 2.160-2.283
Phenakite 7-3/4 2.965
Quartz 7 2.670
Rock-crystal 7