PRECIOUS STONES. 179
best
stones, though the Hungarian locality was known. It is very likely that
many of the supposed Indian specimens were Hungarian ones that found
their way to the West from Constantinople.
The
Hungarian locality is at Czerwenitza, near Eperies (Presova), in Saros.
The Precious Opal here occurs in fissures in a weathered andesitic lava
with other forms of Opal; it was formerly quarried in open workings,
but now a perfect network of levels has burrowed into the mountain. The
largest mass found here is in the Imperial collection at Vienna; it
weighs about 3,000 carats, and is of the size of a man's fist. When the
Opal is in small disseminated patches in the matrix, the whole is
sometimes cut and mounted together, the matrix being oiled to darken
it. Such material is known as " Mother-of-Opal." Cutting the Precious
Opal is a very delicate operation on account of the liability to
breakage of the gem from the numerous flaws. It is partly cut in
Hungary, the operators using a leaden disc, with emery as an abrasive.
The form of cutting employed is nearly always one with a low curved
upper surface without any facets.
These
mines have certainly been known from the fourteenth century. Hungarian
Opals show the finest fire, and their colours deteriorate least with
exposure.
Precious
Opal is also found in a weathered volcanic rock in the west of
Honduras, where it occurs, as in Hungary, as patches in common Opal.
In
Mexico it occurs in the State of Queretaro, north-west of the city of
Mexico, in volcanic rock, and associated with other forms of Opal. The
colours are often intense, but in larger patches than the Hungarian
specimens show, and
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