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DE NATURA FOSSILIUM
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a weaker brilliancy than the true and are scratched by a file. Less cleverly they attempt to produce a false carbunculus by
cementing two pieces of glass together, or better two pieces of quartz,
with a thin layer of minium between them. Minium is called "dragon's
blood" in their workshops. Since both quartz and glass are scratched by
a file this deception is not difficult to detect even when the stone is
in a ring or closed setting. When it is taken out of its setting the
cementing can be seen and one part separated from the other. These gems
are made in a number of different ways and are called doublets. By
using two pieces of glass or quartz with different dyes between them
rogues produce all manner of colored transparent gems. The finest carbunculus, which we call rubinus (spinel),
is made from a piece of Carthaginian garnet and quartz cemented
together with the garnet on top, the quartz on the bottom and a dye
between them. Since the garnet is not scratched by a file this fraud
cannot be detected unless the gem is taken from its setting. In a
similar fashion, according to Pliny, sardonyx is made form ceraunia which
is cemented to gems, sometimes white, sometimes black, sometimes red.
True but worthless gems are sometimes made into valuable gems in this
way, for example, amethystizon from amethyst; diamond from quartz, colorless corundum, or beryllus similar
to quartz. Certain amethysts are perforated and filled with minium or
are deeply engraved and thin sheets of foil cemented beneath them so
that they may be passed as carbunculi. Quartz, corundum and beryllus are
given the hexagonal form of diamond and true diamond is cemented on top
of them since it is a common practice to make diamond doublets. These
artificial diamonds and carbunculi are very similar to true
gems when set in a ring but when they are taken from the setting and
the foils or dyes removed the fraud is apparent. Quartz is scratched
with a file even when set in a ring and can be readily identified in
this way. Moreover, with transparent gems, the portion that receives
the light appears to be brilliant clear, the portion that reflects the
light, if it only reflects it, appears to be even clearer. With all
other gems, of whatever color they may be, they reflect the same color.
The varicolored gems are even more brilliant. This property is best
observed in gems that are angular or rounded like a shield.
Gems
have one, two or more colors as was mentioned in Book One and I shall
review them here in a few words. Many gems have only one color, for
example, quartz, asterios and diamond are colorless; smaragdus, beryllus and prase, green; sapphire, blue; carbunculus and sard, red; sandastros and chrysolithos, yellow. When a gem has two colors these may be distinct or mixed. Several have the colors mixed, aphrodisiace being reddish white, xanthos, yellowish white and the three species of batrachiles, reddish black. Examples of stones with two distinct colors are, apsyctos with reddish veins in a black stone, nasamonites with
black veins in a red stone, heliotrope, with blood-red veins in a
leek-green stone. The opal has three intermingled colors, flame, purple
and green. According to Jacchus the
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