ll the
very brilliant pastes are fairly soft.

DOUBLETS. To give better wearing quality to paste imitations the
_doublet_ was devised. This name is used because the product is in two
parts, a lower or back portion of paste and an upper or top portion of
some cheap but hard genuine stone. Garnet is probably used for this
purpose to a greater extent than any other material, although quartz or
colorless topaz will do very well.

The usual arrangement of the parts can be seen in Fig. 15, the garnet
covering only a part of the upper surface, namely the table part and a
small portion of the sloping surface of the top. In high class doublets
the hard mineral covers the paste to the girdle. (See Fig. 16.) The
color of the garnet does not interfere seriously with that of the paste.

[Illustration: FIG. 15. ONE FORM OF CHEAP DOUBLET.]

If a "diamond" doublet is desired the slice of garnet is made nearly as
thin as paper and it covers only the table of the brilliant. It is thus
practically colorless. A thin slice of red garnet over a green
background is not noticeable, as all the red is absorbed in passing
through the green material beneath. With a blue base, the red upper
layer may give a very slight purple effect. With yellow a slight orange
tint results and of course with a red back no perceptible difference
would result.

[Illustration: FIG. 16. ANOTHER FORM OF DOUBLET.]

The two materials are cemented together, by means of a transparent
waterproof cement. The _triplet_ has already been described in Lesson
XII. It is even better than the doublet and more difficult to detect.
Both the file test and the sunlight-card test serve to detect doublets,
as well as paste imitations, except that in the file test with the fully
protected doublet the _back_ of the stone must be tested with the file,
as the girdle and top are of hard material.

In the sunlight-card test of a doublet (the refraction of garnet being
single like that of glass), single images of the facets will be had on
the card when the sunlight is reflected onto it. A reflection of the
lower or inner surface of the garnet top can be seen also and this
serves to still further identify a doublet or a triplet. The appearance
of this reflection is much like that received on the card from the top
of the table. It is larger than the reflections of the smaller facets
and is but little colored.

TESTS FOR DOUBLETS. A trained eye can also detect a doublet or a triplet
by n