Emerald (Span. esmeralda, Fr. émeraude, Ger. smaragd, Gr. smaragdós), a mineral generally regarded by mineralogists as merely another variety of the same species with the Beryl (q.v.), with which it essentially agrees in composition, crystallisation, &c., differing in scarcely anything but colour. The emerald, which, as a gem, is very highly esteemed, owes its value chiefly to its extremely beautiful velvety green colour. It is composed of about 67-68 per cent. of silica, 15-18 of alumina, 12-14 of glucina, and minute proportions of sesquioxide of chromium, magnesia, and carbonate of lime. Its colour is ascribed chiefly to the oxide of chromium which it contains. Its specific gravity is 2.70-2.76. In hardness it is rather inferior to topaz. The localities in which the emerald is found are very few. The finest have long been brought from Colombia (q.v.), where they are obtained from veins traversing clay-slate, hornblende slate, and granite; and valuable stones also come from the Upper Orinoco, in Venezuela. Stones of inferior quality are found in Europe, imbedded in mica-schist, in the Henbach Valley, in Salzburg. They also occur in the Urals; and some old mines in Upper Egypt have been found to yield them. This gem, known from very early times, was highly prized by the ancients. Pliny states that when Lucullus landed at Alexandria, Ptolemy offered him an emerald set in gold, with his portrait engraved on it. Many wrought emeralds have been found in the ruins of Thebes. Nero, who was near-sighted, looked at the combats of gladiators through an eye-glass of emerald, and concave eye-glasses of emerald seem to have been particularly esteemed among the ancients. As a precious stone, the emerald is rarely without flaw. Its value also depends much on its colour. A very perfect emerald of six carats has been sold for £1000. It appears not improbable that emeralds have been found in the East, in localities not at present known, but the name Emerald or Oriental Emerald is often given to a very rare, beautiful, and precious green variety of Sapphire (q.v.).
EMERALD COPPER is a beautiful and very rare emerald-green crystallised mineral, also called Diopside, found first and chiefly in limestone in the hill of Alтын-Tübe (Altai Mountains), and also obtained in the Urals and the South American Cordilleras. It is composed of about 39 parts silica, 50 protoxide of copper, and 11 water.