The emerald is a green beryl. Color excepted, it is the same as aquamarine, golden, and other beryls. In common with the best of everything in nature, it is the rarest variety of the beryls, and a flawless emerald is more rare than any other precious stone.
Its beautiful color has been attributed by chemists and scientists to various causes. Some have thought it due to the presence of a small quantity of organic matter, but the preponderance of opinion is that the coloring-matter is an oxide of chromium.
Clear and very transparent aquamarine is abundant, but emerald is peculiarly subject to structural defects, flaws, muddiness, and variations of color, and the better the color the more faulty it usually is. A flawless emerald of fine color is almost unknown.
It is a silicate. Over two-thirds of its composition is silica, an oxide of silicon. The remainder is composed of nearly equal parts of alumina and glucina. It crystallizes in the hexagonal system, and takes the form of a six- or twelve-sided prism. It differs from the beryl in that the prisms are usually without striations.
It was known and esteemed by the ancients, though undoubtedly included by them with other green stones under the term " smaragdus." There is evidence, also, that green glass often passed among them for emerald.
At one time very rare in Europe, it became comparatively common after the discovery of Peru. The Spanish marauders robbed that country of great quantities of the precious mineral. One writer says that the ship on which he returned to Spain