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114
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DE NATURA FOSSILIUM
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However,
irrespective of the manner in which gems occur they are never as
brilliant and transparent in the natural state as when polished. They
are first polished with an earth called tripoli.2 This earth
is sprinkled on a lead wheel which is rotated with the right hand while
the gem is held firmly against it with the left. Whether the gem is
polished or engraved it is always cemented, with a variety of pitch, to
the end of a wooden spindle which is held in the hand. In order to give
the gem a higher polish it is held tightly against a wooden wheel
covered with the hide of an elk or some equally thick skin. Thus the
old method of polishing gems on Naxian or Armenian whetstones has been
changed. Artisans usually give an angular shape to certain massive gems
before they are polished. Although the hexagonal smaragdus, carbunculns, sapphire
and even the diamond have natural faces their brilliancy is enhanced
by cutting new facets on them. The dull color of the hexagonal beryl of
India is enlivened by the reflections of the angles. Many facets are
added to European quartz and jaspis in order to make them more
becoming as ornaments. Powdered emery is sprinkled on brass wheels when
these are used instead of lead and first one part of a stone is held
firmly aganist the wheel and then another part until it is faceted. All
gems can be cut with emery powder except diamond which can only be cut
with the diamond powder brought from India by the Lusitanians. Gems are
also suitable for relief engraving and the finest figures are carved
on them. Some gems can be engraved more easily than others, for
example, carnelian, onyx, sardonyx, amethyst, jaspis, molochites and smoky quartz. Certain others stubbornly resist efforts to engrave them, for example, Indian diamond, Scythian and Egyptian smaragdus,' sapphirus* sapphire and the carbuncidus which holds part of the wax in the signet. Archelaus writes that the Carthaginian garnet (carchedonius) can
melt the wax from signets, even the deepest part. Although the diamond
is the most difficult of all stones to engrave this is sometimes done
to conceal a flaw in a prominent part but because of the hardness and
the difficulty in cutting it, the stone is usually hollowed out with
the sharp point of another diamond or with diamond fragments set in
iron. This iron is set in a square hole in a brass shaft. A heavy cord
goes around this shaft and down around a wheel. The engraver, turning
the wheel with the right hand and at the same time rotating the shaft,
applies the gem set in pitch to the diamond point with the left hand.
The diamond which is to be engraved is cemented with pitch to the end
of a wooden spindle. In this way figures are engraved on a gem. Stones
2 A siliceous deposit formed from the shells of diatoms.
3 It is of interest that Agricola places the emerald (Egyptian smaragdus), a relatively soft gem, in this class.
4 In
general Agricola uses this name for lapis-lazuli, a gem which is softer
than any of those mentioned above. This must refer to the true sapphire
which he usually calls cyanus. The name sapphire is of Oriental origin and this may be the first time it is used in its modern sense.
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