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Chalcedony
This is the name given to the microcrystalline varieties of
quartz that form concretionary deposits (partially of organic
origin in the case of jasper). They have been used since time
immemorial both as gems, because of their color, hardness,
and ability to take a good polish, and as precious materials
for the production of ornaments or small sculptures.
The different combinations of colors and patterns
have given rise to a specialized nomenclature that was once
of great importance. The name chalcedony probably comes from
Calcedon or Calchedon, an ancient port on the Sea of Marmara,
in Asia Minor. Ornamental materials were mined in that area,
and it was an active trading center for precious stones of
various types and origins. The Greek khalkedon and Latin charchedonia
do not appear, at least from the description of Pliny the
Elder, to be the same mineral as the modern chalcedony.
Appearance The typical color
is blue whitish-gray, but for ornamental purposes, the types
that have been variously colored by small quantities of other
elements are usually used. These colors can cover the entire
mass, as with jasper, or just a few thin, successive layers,
as with agate and onyx. The most highly prized colors for
the concretionary varieties, which are translucent to semiopaque,
are brownish yellow (sard), red (cornelian), black, green
(chrysoprase), black-and-white or gray-and-white (onyx); and
yellow, red, brownish red or black for jaspers, which are
semiopaque to opaque. All varieties are cut into cabochons,
engraved, or made into seal stones or rounded, polished, and
pierced for necklaces and other items of jewelry. Various
forms of chalcedony were used extensively in the past for
bases and handles of gold items (statuettes, goblets, cruets)
and for stone inlay work. Agate and onyx, with their consecutive
layers of different colors, make excellent material for cameos;
the contrast between the different layers is used to heighten
the relief.
Some variegated pieces are used for the carving
of multicolored figurines similar to those made from jadeite.
The most highly prized variety nowadays is chrysoprase, which
is a bright green color (commonly known as leek green). Distinctive
features The characteristic colors arranged in zones and the
mid-level hardness and exceptional luster of the stones generally
make identification of chalcedony quite easy. It is worth
remembering, however, that it has a slightly lower density
(2.61 g/cm3) and hardness (about 6.5) than other quartzes.
As with all microcrystalline gems, it has only one refractive
index (about 1.53), which is also slightly lower than that
of most quartz.
Occurrence Large amounts of
chalcedony come from Uruguay and the bordering regions of
Brazil, but it is found in many other countries.
In Germany, Idar-Oberstein is famous for agate, although the
term agate apparently comes from Akhates, the Greek name for
a river in Sicily where these stones were found several centuries
sc. Chrysoprase comes mainly from Germany, the Soviet Union,
the United States, Canada, and Brazil.
Value Its value was quite high in antiquity,
when chalcedony was one of the main gems used. Nowadays, it
is fairla low, except for chrysoprase, attractively colored
specimens of which are quite valuable. Objects in chalcedony
o' considerable artistic merit tend to fetch high prices.
Simulants and synthetics In
the past chalcedony was imitated by glass, moulded pieces
even being used to simulate carved stones. It is not produced
synthetically. Because chalcedony is porous, it has long been
the practice to impregnate it with artificial dyestuffs, making
it look like onyx, even where the original color is almost
uniform. This process is facilitated because the porosity
of chalcedony often varies from one layer to another, so that
one layer can absorb color well, whereas the adjoining one
absorbs it little or not at all.
Agate has been artificially colored for sc long
and the procedure is so widespread that it is regardeo as
normal, not fraudulent, practice. Chalcedony is also colored
green to simulate chrysoprase. This practice is considered
fraudulent as the value of chrysoprase depends almost exclusively
on color. Equally fraudulent is intensification of the color
of chrysoprase by the same means.
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