The Emerald: Form and Origin

The emerald, beryl, and aquamarine are, in a scientific point of view, very nearly identical; but in commerce the value of the emerald is infinitely greater than that of the beryl and aquamarine.

The emerald, when it possesses a green tint of a beautiful quality, and when it is entirely hyaline, is one of the most rare and precious of gems. On the contrary, when it appears in semi-transparent crystals of a watery green, it is quite common; indeed there are few granitic mountains where it has not been observed.

The colour so remarkable in the emerald is due to a pretty large quantity, 8 to 9 parts in 100, of oxide of chromium.

The fundamental form of crystals of emerald is the regular six-sided prism. As the side of the base nearly always equals the height, the faces of emerald crystals vary very little from a square.

Another form which frequently occurs is the twelve-sided prism, which is derived directly from the primitive form by the modification of the six vertical edges.

Like the other corundums the emerald is formed chiefly of alumina, but it has a peculiarity which renders it interesting to the chemist, since it contains a considerable proportion, 12 to 15 parts in 100, of a rare body, glucina, the discovery of which is due to the illustrious chemist Vauquelin.

It was for a long time believed that emeralds were always found in connection with granitic rocks; but in 1848 M. Lewy, in the course of his travels in New Grenada, discovered that this opinion was erroneous.

M. Lewy has shown in fact that the most beautiful specimens, those of the mine of Mouza, so far from existing in crystalline rocks, exist, on the contrary, in the best defined secondary formations--in that division of the cretaceous formation to which geologists have given the name of the neocomian. The fossils brought home by M. Lewy leave no room for doubt on this subject.