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SETTING OF GEMS. . 171
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faults—the
momentary detection of these faults being thereby prevented from the
play of the refracting light and the lustre. The color of many gems is
raised by fire, which acts in a peculiar manner on them; thus the. Brazilian topaz assumes
a very fine pale-red color, by burning. The process of effecting this
coloring is very simple, viz : after wrapping the topaz in a sponge,
ignite the same and keep it burning until consumed.
The zircon sometimes assumes a better color after having been subjected to a high degree of heat. Amethysts having
dark spots may be calcined for a short time in a crucible containing
sand and iron filings, under which process they mostly lose those
faults; but if exposed to an excess of heat, they will lose their color
altogether, and become as white as quartz. The Oriental carnelian assumes,
after burning, a fine color, and in Hindostan those carnelians which
are found detached in the mines are cut up and burnt on the spot. Very
fine cracks are sometimes produced in mounting stones, which may be
repaired and concealed successfully by means of garlic juice. When
stones are broken by the same operation they may be cemented by gum
mastic.
SETTING OF GEMS.
The gems are generally fastened or set at the girdle in a box or rim of metal: limpid and faultless gems are always set d jour, i.
e., without backs, since they appear then to the best advantage, and if
the gem is intended to display its full size and color, the djour setting is only fastened by small shanks or claws. The good setting of a gem very much
increases its value and beauty. The material for mounting the limpid
gems is silver, which displays them to more advantage than gold. In
order to increase the color or lustre of large gems, they are often
surrounded by
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