Gems - Chrysoberyl
( Originally Published 1919 )
(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 BeAl2O4, 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. 2). Cat's-eyes (Plate XXIX, Fig. 13) have often also a brownish shade of green. The bluish green and dark olive-green stones known as alexandrite (Plate XXVI I, Figs. 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, xpvvos, golden, and Bnpunnos, beryl, and etymologically more correctly defines the lighter-coloured stones, which were, indeed, at one time the only kind known. Chrysolite from , golden, and , 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 kuua, wave, and , 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 P757, 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, i 3 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 81 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 1 o 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 L2 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, 80 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 631 and 28 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 43 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 XXVI I, Fig. 2. 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 26 carats, are also exhibited in the Mineral Gallery of the British Museum (Natural History). The former is illustrated in Plate XXVII, Figs. I I, 13, as seen in daylight and in artificial light.