CHRYSOBERYL
(_Chrysolite_, _Cat’s-Eye_, _Cymophane_, _Alexandrite_)
Chrysoberyl has at times enjoyed fleeting popularity on account of the
excellent cat’s-eyes cut from the fibrous stones, and in the form of
alexandrite it meets with a steadier, if still limited, demand. It is a
gem-stone that is seldom met with in ordinary jewellery, although its
considerable hardness befits it for all such purposes.
Chrysoberyl is in composition an aluminate of beryllium corresponding
to the formula BeAl_{2}O_{4}, and is therefore closely akin to spinel.
It usually contains some ferric and chromic oxides in place of alumina,
and ferrous oxide in place of beryllia, and it is to these accessory
constituents that its tints are due. Other gem-stones containing the
uncommon element beryllium are phenakite and beryl. Pale yellowish
green, the commonest colour, is supposed to be caused by ferrous
oxide; such stones are known to jewellers as chrysolite (Plate XXVII,
Fig. 12). Cat’s-eyes (Plate XXIX, Fig. 1) have often also a brownish
shade of green. The bluish green and dark olive-green stones known
as alexandrite (Plate XXVII, Figs. 11, 13) differ in appearance so
markedly from their fairer sisters that their common parentage seems
almost incredible. The dull fires that glow within them, and the
curious change that comes over them at night, add a touch of mystery to
these dark stones. Chromic oxide is held responsible for their colour.
The cat’s-eyes are, of course, always cut _en cabochon_, but otherwise
chrysoberyl is faceted.
The name of the species is composed of two Greek words, χρυσός, golden,
and βήρυλλος, beryl, and etymologically more correctly defines the
lighter-coloured stones, which were, indeed, at one time the only kind
known. Chrysolite from χρυσὁς, golden, and λίθος, stone, has much the
same significance. This name is preferred by jewellers, but in science
it is applied to an entirely different species, which is known in
jewellery as peridot. Cymophane, from κῦμα, wave, and φαίνειν, appear,
refers to the peculiar opalescence characteristic of cat’s-eyes; it
is sometimes used to designate these stones, but does not find a
place within the vocabulary of jewellery. Alexandrite is named after
Alexander II, Czar of Russia, because it first came to light on his
birthday. That circumstance, coupled with its display of the national
colours, green and red, and its at one time restriction to the mining
district near Ekaterinburg, renders it dear to the heart of all loyal
Russians.
Chrysoberyl crystallizes in the orthorhombic system, and occurs in
rather dull, complex crystals, which are sometimes so remarkably
twinned, especially in the variety called alexandrite, as to simulate
hexagonal crystals. In keeping with the crystalline symmetry it
is doubly refractive and biaxial, having two directions of single
refraction. The least and the greatest of the principal indices of
refraction may have any values between 1·742 and 1·749, and 1·750 and
1·757, respectively, the maximum amount of double refraction remaining
always the same, namely, 0·009. The mean principal refractive index
is close to the least; the sign of the double refraction is therefore
positive, and the shadow-edge corresponding to the lower index, as
seen in the refractometer, has little, if any, perceptible motion
when the stone is rotated. The converse is the case with corundum;
the sign is negative, and it is the shadow-edge corresponding to the
greater refractive index that remains unaltered in position on rotation
of the stone. This test would suffice to separate chrysoberyl from
yellow corundum, even if the refractive indices of the former were
not sensibly lower than those of the latter. Also, the dichroism of
chrysolite is stronger than that of yellow sapphires. In alexandrite
this phenomenon is most prominent; the absorptive tints, columbine-red,
orange, and emerald-green, corresponding to the three principal
optical directions, are in striking contrast, and the first differs
so much from the intrinsic colour of the stone as to be obvious to
the unaided eye, and is the cause of the red tints visible in a cut
stone. The curious change in colour of alexandrite, from leaf-green to
raspberry-red, that takes place when the stone is seen by artificial
light, is due to a different cause, as has been pointed out above (p.
54). The effect is illustrated by Figs. 11, 13 on Plate XXVII, which
represent a fine Ceylon stone as seen by daylight and artificial light;
the influence of dichroism may be noticed in the former picture. The
specific gravity of chrysoberyl varies from 3·68 to 3·78. In hardness
this species ranks above spinel and comes next to corundum, being
given the symbol 8½ on Mohs’s scale. Certain stones contain a multitude
of microscopic channels arranged in parallel position. When the stones
are cut with their rounded surface parallel to the channels, a broadish
band of light is visible running across the stone at right angles
to them, and suggests the pupil of a cat’s eye, whence the common
name for the stones. The fact that the channels are hollow causes an
opalescence, which is absent from the quartz cat’s-eye.
The most important locality for the yellowish chrysoberyl is the rich
district of Minas Novas, Minas Geraes, Brazil, where it occurs in the
form of pebbles, and excellent material is also supplied by Ceylon,
in both crystals and rounded pebbles. Other places for chrysolite are
Haddam, Connecticut, and Greenfield, Saratoga County, New York, in
the United States, and recently in the gem-gravels near the Somabula
Forest, Rhodesia. Ceylon supplies some of the best cat’s-eyes.
Alexandrite was first discovered, as already stated, at the emerald
mines near Ekaterinburg, in the Urals; but the supply is now nearly
exhausted. A poorer quality comes from Takowaja, also in the Urals.
Good alexandrite has come to light in Ceylon, and most of the stones
that are placed on the market at the present day have emanated from
that island. The Ceylon stones reach a considerable size, often as much
as from 10 to 20 carats in weight; the Russian stones have a better
colour and are more beautiful, but they are less transparent, and
rarely exceed a carat in weight. Good chrysolite may be worth from 10s.
to £2 a carat, and cat’s-eye runs from £1 to £4 a carat, depending
upon the quality. Alexandrites meet with a steady demand in Russia, and
fine stones are scarce; flawless stones about a carat in weight are
worth as much as £30 a carat, and even quite ordinary stones fetch £4 a
carat.
From Ceylon, that interesting home of gems, have originated some
magnificent chrysoberyls, including a superb chrysolite, 80¾ carats
in weight, and another, a splendid brownish yellow in colour and
very even in tint, and two large alexandrites, green in daylight and
a rich red by night, weighing 63⅜ and 28-23/32 carats. The finest
cut chrysolite existing is probably the one exhibited in the Mineral
Gallery of the British Museum (Natural History). Absolutely flawless
and weighing 43¾ carats, it was formerly contained in the famous Hope
collection, and is described on page 56 and figured on Plate XXI of
the catalogue prepared by B. Hertz, which was published in 1839; the
weight there given includes the brilliants and the ring in which it
was mounted. It is shown, about actual size, in Plate XXVII, Fig. 12.
A magnificent cat’s-eye, 35·5 by 35 mm. in size, which also formed
part of the Hope collection, was included in the crown jewels taken
from the King of Kandy in 1815. The crystalline markings in the cut
stone are so arranged that the lower half shows an altar overhung by a
torch. The stone has been famous in Ceylon for many ages. It was set
in gold with rubies cut _en cabochon_. Two fine Ceylon alexandrites of
exceptional merit, weighing 42 and 26¾ carats, are also exhibited in
the Mineral Gallery of the British Museum (Natural History). The former
is illustrated in Plate XXVII, Figs. 11, 13, as seen in daylight and in
artificial light.