EMERALD, Smaragdus, Schmaragd, Emeraude verte. In mineralogy this includes two subspecies; the emerald properly so called, or the precious emerald, comprehending the transparent and beautiful green-coloured varieties, and the beryl, which has already been described under its appropriate head.

The emerald always occurs crystallized, its most common form being that of a regular six-sided prism, occasionally modified by truncations on the terminal edges. It is in general well characterized by that pure and lively green colour which has hence received the name of emerald green ; its hue, however, varies somewhat, inclining at times to verdigris or grass green, and frequently becoming rather pale. This fine colour is occasioned by the presence of a minute portion of the oxide of chrome, to which, indeed, may be attributed the principal distinction between emerald and the beryl; the mountain-green and yellow colours of the latter being produced by an admixture of iron in different states of oxidation. The most magnificent crystals of emerald occur in veins of magnesian carbonate of lime, which traverse a hornblende rock in the mine of Muso, near Santa Fé de Bogota, in Peru. A hexagonal prism from this locality, upwards of two inches in length, and measuring across its three diameters ¾, 2⅛, and 1⅞ inches, is now in the possession of the Duke of Devonshire: it weighs eight ounces and eighteen dwts. and, though imperfect for the lapidary’s purposes, from the numerous flaws it contains, is unquestionably the finest crystallized emerald at present in Great Britain. Less distinct varieties, generally of muddy emerald-green colours, occur imbedded in mica slate in the Pinzgau valley, Saltzburg; also in mica-schiste at Mount Zabara in Upper Egypt, a locality whence the ancients are believed to have obtained their emeralds.

The Peruvian emerald, according to the analyses of Klaproth and Vauquelin, contains,
[table]
[7:8:682]

Its specific gravity ranges between 2∙6 and 2∙8, and its hardness between 7∙5 and 8Ό of the scale of Mohs; that is, it scratches quartz, and is very little inferior to topaz in hardness. It possesses a vitreous lustre, more or less shining, and a conchoidal, uneven fracture. By friction it becomes electric, and before the blowpipe with borax fuses into a transparent colourless glass. But it is as a gem inferior only to the diamond and the ruby that this mineral is so highly prized and esteemed. Its colour, which is perfectly distinct from that of any other gem, is the richest, and presents by daylight the most pleasing hue imaginable. An emerald, therefore, possessing considerable surface, and free from flaws and impurities, has always been very highly prized, and, according to Pliny (lib. xxxvii.), even in his time commanded immense sums. Haüy describes one in the tiara of the sovereign pontiff when in Paris in 1804, consisting of a cylinder about two inches long by two and a quarter broad, which is supposed to have been brought from Africa, as it bears the name of Julius II. who flourished anterior even to the conquest of Peru. The most magnificent cut emerald in this country is that in the possession of Mr Hope of London: it weighs six ounces, is perfect in colour and transparency, and cost L.500. It is believed to be from Coimbetoor.

Emeralds are cut and polished with facility. The usual form given them is that of a square table with the edges replaced, the lower surface being cut into facets parallel to their sides. When fine they are always set without a foil, and, in consequence of their brilliancy being somewhat impaired by candle light, are generally surrounded with small diamonds or pearls, which on such occasions add considerably to their lustre and effect. This gem has been imitated by the manufacturers of paste stones in France with very considerable success, the colouring matter which they use being the oxide of chrome.