PERIDOT
The beautiful bottle-green stone, which from its delicate tint has
earned from appreciative admirers the poetical _sobriquet_ of the
evening emerald, and which has during recent years crept into popular
favour and now graces much of the more artistic jewellery, is named
as a gem-stone peridot—a word long in use among French jewellers, the
origin and meaning of which has been forgotten—but is known to science
either as olivine, on account of the olive-green colour sometimes
characterizing it, or as chrysolite. It is of interest to note that the
last word, derived from χρυσός, golden, and λίθος, stone, was in use at
the time of Pliny, but was employed for topaz and other yellow stones,
while his topaz, curiously enough, designated the modern peridot (cf.
p. 199), an inversion that has occurred in other words. The true
olivine must not be confused with the jewellers’ ‘olivine,’ which is a
green garnet from the Ural Mountains (p. 217). Peridot is comparatively
soft, the hardness varying from 6½ to 7 on Mohs’s scale, and is
suitable only for articles which are not likely to be scratched; the
polish of a peridot worn in a ring would soon deteriorate. The choicest
stones are in colour a lovely bottle-green (Plate XXIX, Fig. 2) of
various depths; the olive-green stones (Plate XXIX, Fig. 3) cannot
compare with their sisters in attractiveness. The step form of cutting
is considered the best for peridot, but it is sometimes cut round or
oval in shape, with brilliant-cut fronts.
Peridot is a silicate of magnesium and iron, corresponding to the
formula (Mg,Fe)_{2}SiO_{4}, ferrous iron, therefore, replacing
magnesia. To the ferrous iron it is indebted for its colour, the
pure magnesium silicate being almost colourless, and the olive tint
arises from the oxidation of the iron. The latitude in the composition
resulting from this replacement is evinced in the considerable range
that has been observed in the physical characters, but the crystalline
symmetry persists unaltered; the lower values correspond to the stones
that are usually met with as gems. Peridot belongs to the orthorhombic
system of crystalline symmetry, and the crystals, which display a
large number of faces, are prismatic in form and generally somewhat
flattened. The stones, however, that come into the market for cutting
as gems are rarely unbroken. The dichroism is rather faint, one of the
twin colours being slightly more yellowish than the other, but it is
more pronounced in the olive-tinted stones. The values of the least
and greatest of the principal indices of refraction vary greatly,
from 1·650 and 1·683 to 1·668 and 1·701, but the double refraction,
amounting to 0·033, remains unaffected. Peridot, though surpassed by
sphene in extent of double refraction, easily excels all the ordinary
gem-stones in this respect, and this character is readily recognizable
in a cut stone by the apparent doubling of the opposite edges when
viewed through the table facet (cf. p. 41). An equally large
variation occurs in the specific gravity, namely, from 3·3 to 3·5.
[Illustration: _PLATE XXVII_
1. RUBELLITE
2. TOURMALINE
3. BALAS-RUBY
4. BLUE SPINEL
5. QUARTZ
6. WHITE OPAL
7. AMETHYST
8. TOURMALINE
9. BLACK OPAL
10. FIRE OPAL
11. ALEXANDRITE (_By daylight_)
12. CHRYSOBERYL
13. ALEXANDRITE (_By artificial light_)
GEM-STONES]
Peridots of deep bottle-green hue command moderate prices at the
present day, about 30s. a carat being asked for large stones; the paler
tinted stones run down to a few shillings a carat. The rate per carat
may be very much larger for stones of exceptional size and quality.
Olivine, to use the ordinary mineralogical term, is a common and
important constituent of certain kinds of igneous rocks, and it is
also found in those strange bodies, meteorites, which come to us
from outer cosmical space. Except in basaltic lavas, it occurs in
grains and rarely in well-shaped crystals. Stones that are large and
transparent enough for cutting purposes come almost entirely from the
island Zebirget or St. John situated on the west coast of the Red Sea,
opposite to the port of Berenice. This island belongs to the Khedive of
Egypt, and is at present leased to a French syndicate. It is believed
to be the same as the mysterious island which produced the ‘topaz’
of Pliny’s time. Magnificent stones have been discovered here, rich
green in colour, and 20 to 30, and occasionally as much as 80, carats
in weight when cut; a rough mass attained to the large weight of 190
carats. Pretty, light-green stones are supplied by Queensland, and
peridots of a less pleasing dark-yellowish shade of green, and without
any sign of crystal form, have during recent years come from North
America. Stones rather similar to those from Queensland have latterly
been found in the Bernardino Valley in Upper Burma, not far from the
ruby mines.