BERYL
(Emerald, Aquamarine, Morganite)
THE species to be considered in this chapter includes the varieties emerald and aquamarine, as well as what jewellers understand by beryl. It has many incontestable claims on the attention of all lovers of the beautiful in precious stones. The peerless emerald ([Plate I], Fig. 5), which in its verdant beauty recalls the exquisite lawns that grace the courts and quadrangles of our older seats of learning, ranks to-day as the most costly of jewels. Its sister stone, the lovely aquamarine ([Plate I], Fig. 4), which seems to have come direct from some mermaid’s treasure-house in the depths of a summer sea, has charms not to be denied. Pliny, speaking of this species, truly says, “There is not a colour more pleasing to the eye”; yet he knew only the comparatively inferior stones from Egypt, and possibly from the Ural Mountains. Emeralds are favourite ring-stones, and would, no doubt, be equally coveted for larger articles of jewellery did not the excessive cost forbid, and nothing could be more attractive for a central stone than a choice aquamarine of deep blue-green hue. Emeralds are usually step-cut, though Indian lapidaries often favour the en cabochon form; aquamarines, on the other hand, are brilliant-cut in front and step-cut at the back.
Beryl, to use the name by which the species is known to science, is essentially a silicate of aluminium and beryllium corresponding to the formula, Be3Al2(SiO3)6. The beryllia is often partially replaced by small amounts of the alkaline earths, caesia, potash, soda, and lithia, varying from about 1½ per cent. in beryl from Mesa Grande to nearly 5 in that from Pala and Madagascar, and over 6, of which 3·6 is caesia, in beryl from Hebron, Maine; also, as usual, chromic and ferric oxides take the place of a little alumina; from 1 to 2 per cent. of water has been found in emerald. The element beryllium was, as its name suggests, first discovered in a specimen of this species, the discovery being made in 1798 by the chemist Vauquelin; it is also known as glucinum in allusion to the sweet taste of its salts.
When pure, beryl is colourless, but it is rarely, if ever, free from a tinge of blue or green. The colour is usually some shade of green—grass-green, of that characteristic tint which is in consequence known as emerald-green, or blue-green, yellowish green ([Plate I], Fig. 6), and sometimes yellow, pink, and rose-red. The peculiar colour of emerald is supposed to be caused by chromic oxide, small quantities of which have been detected in it by chemical analysis; moreover, experiment shows that glass containing the same percentage amount of chromic oxide assumes the same splendid hue. Emerald, on being heated, loses water, but retains its colour unimpaired, which cannot therefore be due, as has been suggested, to organic matter. The term aquamarine is applied to the deep sea-green and blue-green stones, and jewellers restrict the term beryl to paler shades and generally other colours, such as yellow, golden, and pink, but Kunz has recently proposed the name morganite to distinguish the beautiful rose beryl such as is found in Madagascar. The varying shades of aquamarine are due to the influence of the alkaline earths modified by the presence of ferric oxide or chromic oxide; the beautiful blushing hue of morganite is no doubt caused by lithia.
Fig. 70.—Emerald
Crystal.
The name of the species is derived from the Greek βήρυλλος, an ancient word, the meaning of which has been lost in the mists of time. The Greek word denoted the same species in part as that now understood by the name. Emerald is derived from a Persian word which appeared in Greek as σμάραγδος, and in Latin as smaragdus; it originally denoted chrysocolla, or similar green stone, but was transferred upon the introduction of the deep-green beryl from Upper Egypt. The name aquamarine was suggested by Pliny’s exceedingly happy description of the stones “which imitate the greenness of the clear sea,” although it was not actually used by him. That emerald and beryl were one species was suspected by Pliny, but the identity was not definitely established till about a century ago. Morganite is named after John Pierpont Morgan.
The natural crystals have the form of a six-sided prism, and in the case of emerald (Fig. 70, and [Plate I], Fig. 8) invariably, if whole, end in a single face at right angles to the length of the prism; aquamarines have in addition a number of small inclined faces, and stones from both Russia and Brazil often taper owing to the effects of corrosion. The sixfold character of the crystalline symmetry necessarily entails that the double refraction, which is small in amount, 0·006, is uniaxial in character, and, since the ordinary is greater than the extraordinary refractive index, it is negative in sign. The values of the indices range between 1·567 and 1·590, and 1·572 and 1·598 respectively, in the two cases, the pink beryl possessing the highest values. The dichroism is distinct in the South American emerald, the twin colours being yellowish and bluish green, but otherwise is rather faint. The specific gravity varies between 2·69 and 2·79, and is therefore a little higher than that of quartz. If, therefore, a beryl and a quartz be floating together in a tube containing a suitable heavy liquid, the former will always be at a sensibly lower level (cf. Fig. 32). The hardness varies from 7½ to 8, emerald being a little softer than the other varieties. There is no cleavage, but like most gem-stones beryl is very brittle, and can easily be fractured. Stones rendered cloudy by fissures are termed ‘mossy.’ When heated before the blowpipe beryl is fusible with difficulty; it resists the attack of hydrofluoric acid as well as of ordinary acids.
In all probability the whole of the emeralds known in ancient times came from the so-called Cleopatra emerald mines in Upper Egypt. For some reason they were abandoned, and their position was so completely lost that in the Middle Ages it was maintained that emeralds had never been found in Egypt at all, but had come from America by way of the East. All doubts were set at rest by the re-discovery of the mines early last century by Cailliaud, who had been sent by the Viceroy of Egypt to search for them. They were, however, not much worked, and after a few years were closed again, and were re-opened only about ten years ago. The principal mines are at Jebel Zabara and at Jebel Sikait in northern Etbai, about 10 miles (16 km.) apart and distant about 15 miles (24 km.) from the Red Sea, lying in the range of mountains that run for a long distance parallel to the west coast of the Red Sea and rise to over 1800 feet (550 m.) above sea-level. There are numerous signs of considerable, but primitive, workings at distinct periods. Both emeralds and beryls are found in micaceous and talcose schists. The emeralds are not of very good quality, being cloudy and rather light in colour. Finer emeralds have been found in a dark mica-schist, together with other beryllium minerals, chrysoberyl and phenakite, and also topaz and tourmaline on the Asiatic side of the Ural Mountains, near the Takowaja River, which flows into the Bolshoi Reft River, one of the larger tributaries of the Pyschma River, about fifty miles (80 km.) east of Ekaterinburg, a town which is chiefly concerned with the mining and cutting of gem-stones. The mine was accidentally discovered by a peasant, who noticed a few green stones at the foot of an uprooted tree in 1830. Two years later the mine was regularly worked, and remained open for twenty years, when it was closed. It has recently been re-opened owing to the high rates obtaining for emeralds. Very large crystals have been produced here, but in colour they are much inferior to the South American stones; small Siberian emeralds, on the other hand, are of better colour than small South American emeralds, the latter being not so deep in tint. Emeralds have been found in a similar kind of schist at Habachtal, in the Salzburg Alps. About thirty years ago well-formed green stones were discovered with hiddenite at Stony Point, Alexander County, in North Carolina, but not much gem material has come to light.