with
gold; faulty specimens are generally chosen, as the defects can thus be
concealed. The Shah of Persia is supposed to have in his possession all
the finest gems, allowing those only of inferior quality to leave the
country.
The
turquoise, in ancient times called the callaite, was well known to the
Greeks and Romans. Pliny notices its changing colour from gradual
decomposition, and also shares in the oriental belief that this gem was
lucky, and brought health and fortune to the wearer. Several antique
cameos and intaglios cut in this material are extant in the Vatican,
some of which still retain their colour. Fragments of turquoise, which
appear to have been parts of amulets, are frequently met with in the
ruins of ancient towns in Egypt.
THE OPAL.
This
magnificent gem is composed of silica in an amorphous state, mixed with
water, and is in reality the same mineral as quartz, with the addition
of 6 or 7 per cent, of water: it never occurs in a crystallized form,
it has a vitreous lustre inclining to resinous, is numbered in the
table of hardness from 5.5 to 6.5, is scratched by quartz, and has a specific gravity varying from 1.9 to 2.3.
There
are many varieties,—the " noble," or precious opal used by jewellers;
the "fire," or reddish opal, which has also occasionally a fine play of
colours; the so-called common opal; the semi- or half-opal; the hydro-
