ONYX,
SARD, AND SARDONYX (CHALCEDONY).
Onyx,
sard and sardonyx are all varieties of the microcrystalline
quartz, chalcedony. Onyx is similar to agate but it has
straight rather than curved bands. These may be brown and
white or black and white. Sard is a brownish red variety,
also similar to agate. Sardonyx, a blend of sard and onyx,
has the straight white bands of onyx and the brownish red of
sard. All three varieties are curved as small sculptures and
intaglios, or may be polished, tumbled, or cut as beads.
They are renowned as excellent materials for inlay work.
Since Ancient Egyptian times, onyx has been stained to
improve or change its colour. Much onyx has been produced by
soaking agate in a sugar solution, then heating it in
sulphuric acid to carbonize the sugar particles. Sard may be
imitated by saturating chalcedony with an iron solution.
Occurrence
Found
worldwide, they are formed by the deposition of silica in
gas cavities in lavas, which results in the distinctive
bands
REMARK
Onyx
seals were very popular with the Romans, who carved the
pattern of the seal in negative relief to give a raised
print. They often used stoned with several layers, each of a
different colour, which were then individually carved to
produce a different pattern in each layer.
FLOWER
CAMEO
This cameo
was worked from a single piece on onyx. The dark, opaque
layer has been carved away in the shape of a flower to
reveal the pale layer beneath.
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SARDONYX
CAMEO
In this
intricately carved cameo, the pattern of a
women's head and winged dragon has been cut from
three different layers-dark brown, A laurel
garland is carved just inside the raised rim. |
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STRAIGHT
SEAL
The straight
layers of onyx have been exposed to dramatic
effect in this seal, an ornament popular with
the Romans.
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OVAL
POLISHED
SARD
Stone is
translucent, with patchy colour.
Characteristic
brownish red colour |
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