s are commonly cut in
the mixed cut. Recently it has become common to polish the tops of
colored stones with a smooth unfacetted, slightly convex surface, the
back being facetted in either the brilliant or the step arrangement.
Such stones are said to have a "_buffed top_." They are less expensive
to cut than fully facetted stones and do not have the snappy brilliancy
of the latter. They do, however, show off the intrinsic color of the
material very well.

LESSON XXV

IMITATIONS OF PRECIOUS STONES

"PASTE" GEMS. Large volumes have been written on paste jewels,
especially on antique pastes. Contrary to a prevailing belief, the paste
gem is not a recent invention. People frequently say when told that
their gems are false, "But it is a very old piece, it must be genuine."
The great age of a jewel should rather lead to suspicion that it was not
genuine than give confidence that a true gem was assured. The Egyptians
and Romans were skillful makers of glass of the sort used in imitating
gems and some of the old pastes were very hard or else have become so
with age.

Glass of one variety or another makes the most convincing sort of
imitation precious stones. The term "paste" as applied to glass
imitations is said to come from the Italian _pasta_ meaning dough, and
it suggests the softness of the material. Most pastes are mainly lead
glass. As we saw in Lesson XVIII., on the chemical composition of the
gems, many of them are silicates of metals. Now glasses are also
silicates of various metals, but unlike gem minerals the glasses are not
crystalline but rather amorphous, that is, without definite geometric
form or definite internal arrangement.

The optical properties of the various glasses vary chiefly with their
densities, and the denser the material the higher the refractive index
and the greater the dispersion. Thus to get the best results in
imitation stones they should be made of very heavy glass. The dense
flint glass (chiefly a silicate of potassium and lead) which is used for
cut glass ware illustrates admirably the optical properties of the heavy
glasses. By using even more lead a still denser glass may be had, with
even a greater brilliancy.

Unfortunately the addition of lead or other heavy metals (such as
thallium) makes the product very soft and also very subject to attack by
gases such as are always present in the atmosphere of cities. This
softness causes the stones to scratch readily so that whe