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Beryl

BERYL, a mineral containing beryllium and aluminium in the form of a silicate; its formula is Be3Al2Si6O18. The species includes the emerald (q.v.), the aquamarine (q.v.) and other transparent varieties known as "precious beryl," with certain coarse varieties unfit for use as gem-stones. The name comes from the Gr., a word of uncertain etymology applied to the beryl and probably several other gems. It is notable that the relation of the emerald to the beryl, though proved only by chemical analysis, was conjectured at least as far back as the time of Pliny.

Beryl crystallizes in the hexagonal system, usually taking the form of long six-sided prisms, striated vertically and terminated with the basal plane, sometimes associated with various pyramidal faces (see fig.). It cleaves rather imperfectly parallel to the base. The colour of beryl may be blue, green, yellow, brown or rarely pink; while in some cases the mineral is colourless. The specific gravity is about 2.7, and the hardness 7.5 to 8, so that for a gem-stone beryl is comparatively soft. Whilst the gem-varieties are transparent, the coarse beryl may be opaque. The transparent crystals are pleochroic - a character well marked in emerald.

Crystal of beryl.

Beryl was much prized as a gem-stone by the ancients, and Greek intaglios of very fine workmanship are extant. The Roman jewellers, taking advantage of the columnar form of the natural crystal, worked it into long cylinders for ear-pendants. It was a favourite stone with the artists of the Renaissance, but in modern times has lost popularity, except in the form of emerald, which remains one of the most valued gem-stones. It is notable that English lapidaries of the 18th century often included the sard under the term beryl - a practice which has led to some confusion in the nomenclature of engraved gems.

Beryl occurs as an accessory constituent of many granitic rocks, especially in veins of pegmatite, whilst it is found also in gneiss and in mica-schist. Rolled pebbles of beryl occur, with topaz, in Brazil, especially in the province of Minas Geraes. Crystals are found in drusy cavities in granite in the Urals, notably near Mursinka; in the Altai Mountains, which have yielded very long prismatic crystals; and in the mining district of Nerchinsk in Siberia, principally in the Adun-Chalon range, where beryl occurs in veins of topaz-rock piercing granite. Among European localities may be mentioned Elba, good crystals being occasionally found in the tourmaline-granite of San Piero. In Ireland excellent crystals of beryl occur in druses of the granite of the Mourne Mountains in Co. Down, and others less fine are found in the highlands of Donegal, whilst the mineral is also known from the Leinster granite. It occurs likewise in the granite of the Grampians in Scotland, and is not unknown in Cornwall, specimens having been found, with topaz, apatite, etc., in joints of the granite of St Michael's Mount.

Many localities in the United States yield beryl, sometimes sufficiently fine to be cut as a gem. It is found, for example, at Hiddenite and elsewhere in Alexander county, N.C.; at Haddam and Monroe, Conn.; at Stoneham and at Albany, in Oxford county, Maine; at Royalston, Mass.; and at Mt. Antero, Colorado, where it occurs with phenacite. Beryl of beautiful pink colour occurs in San Diego county, California. Coarse beryl, much rifted, is found in crystals of very large size at Grafton and Acworth, N.H.; a crystal from Grafton weighing more than 2 tons. A colourless beryl from Goshen, Mass., has been called Goshenite; whilst crystals of coarse yellow beryl from Rubislaw quarry in Aberdeenshire, Scotland, have been termed Davidsonite.

Beryl suffers alteration by weathering, and may thus pass into kaolin and mica.

(F. W. R.*)

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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