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Ch. 14: Emerald, Aquamarine, Beryl

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	264 PRECIOUS STONES.

	emeralds consist of these layers mounted without any artificial cutting or polishing. When seen at right angles to the optic axes, the emerald presents one image of greenish yellow and another of greenish blue tints. The color of this gem, which constitutes one of its principal charms, is a lively grass-green without admixture of any other hues, and has generally been supposed to be due to the oxide of chrome, though this theory is not universally adopted ; some scientists referring it to copper, some to iron, and others to an entirely different cause. It is extremely difficult, says Mr. Rudler, to determine the precise nature of the coloring matter present in gems, since it is so intense that the smallest possible quantity is sufficient to give them a decided tint. This question gave rise to some very interesting experiments made by M. Lewy, in 1848, which led him to conclude the color was derived from some organic matter similar to the coloring substance of the green leaves of plants, called chlorophyl. He assumed that the emerald loses its hue by heat, while chromic oxide is a stable pigment, and ought not to be affected by it. This experimenter burned it in oxygen, and found that carbonic acid was produced, as when a diamond is subjected to the same operation ; therefore he believed it must contain carbon. The conclusion reached after repeated experiments was that the coloring substance consists of a compound of carbon and hydrogen, resulting from organic matter. Emeralds from Muzo, South America, have been found in a fossiliferous limestone of a bituminous nature ; therefore their color, it is reasoned, must have been caused by the decomposition of animal matter, similar to that found in vegetables. M. Lewy obtained by his analyses a certain per cent of water, which led him to infer that these gems were

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