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of
glass rarely over 5. An aluminum pencil also affords a safe means of
testing hardness. Drawn over glass it leaves a white, silvery line, but
on hard gems little if any mark. In respect to color, luster, and even
specific gravity, glass may be made to imitate almost any gem. Even
natural looking flaws can be made in a glass imitation by dexterous
hammer blows. Nevertheless glass can often, though not always, be
distinguished from a mineral by the fact that in a piece of glass
minute air bubbles may be seen on examining it with a lens. These
bubbles generally differ in shape and number from any found in natural
minerals. Glass also has a characteristic conchoidal fracture not quite
like that usual to minerals. True gems are colder to the touch than
glass as a rule, although glass is colder than such substances as jet,
amber, and pearl, for which it is often substituted. The colder feeling
of true gems comes from their being better conductors of heat than
glass, so that they take away warmth from the hand more rapidly. For
the same reason most true gems when breathed upon acquire a thicker
coat-ing of moisture than glass and lose it more quickly
than does that substance. In the application of these simple tests
jewelers often become very skilful, and if the stones are not too small
can pick out a diamond, sapphire, or other gem from a whole bagful of
glass imitations by the above distinctions alone. When in the rough, a
useful distinction of glass from most gems is to be found in the easy
fusibility of the former before the blowpipe. While most gems are
practically infusible in this way, glass is easily fused, and hence the
trial of a splinter of the substance before the blowpipe affords a
test of value. The distinction of glass from minerals by an observation
of their behavior in polarized light can be made without injury to the
substance tested, and with reliable results. To be sure, the
distinction of glass from diamond, spinel-ruby, or other singly
refracting gem, cannot be made in this way; but when the stone is
doubly refracting, as is the case with the majority of species, such
investigation affords one of the surest and most convenient means of
identification. The use of the dichroscope or polarizing microscope for
this purpose has already been explained.
The
glass used for making imitation gems is usually one having a high
percentage of lead in its composition. The lead makes it soft but gives
it great brilliancy. The glass is usually known as paste, or strass,
the latter name being from the inventor, Strass of Strassburg, who
invented the mixture during the seventeenth century. Uncolored it
affords a good imitation of the diamond, and when colored with various
metallic oxides, remarkably accurate likenesses of different gems can
be obtained.
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