The coloring matter of the emerald seems to be derived from the decomposition of the remains of animals who have lived in a bygone age, and whose remains are now found fossilized in the rock which forms the matrix of the gem. This rock in Granada is a black limestone, with white veins containing ammonites. Specimens of these rocks, exhibiting fragments of emeralds _in situ_ and also ammonites, are to be seen in the mineralogical gallery of the Jardin des Plantes in Paris. Lewy believes that the beautiful tint of these gems is produced by an organic substance, which he considers to be a carburet of hydrogen, similar to that called chlorophyll, which constitutes the coloring matter of the leaves of plants; and he has shown that the emeralds of the darkest hue, which contain the greatest amount of organic matter, lose their color completely at a low red heat, and become opaque and white; while minerals and pastes which are well known to be colored by chromium, like the green garnets (the lime-chrome garnets) of Siberia, are unchanged in hue by the action of heat. At the present time the composition of the emerald is supposed to be a silicate of alumina and glucina, with traces of organic matter and also other earths and oxides; but silica, alumina, and glucina are the principal component parts. It resembles quartz in some of its physical properties, having a specific gravity of 2.6 to 2.7, and a refractive energy of 1.58, but its degree of hardness is slightly greater, ranging from 7.5 to 8.0, while that of quartz is but 7.0. The Peruvians maintain that the emerald ripens and deepens in color after having been mined and exposed to the air and light. Whether this assertion has been corroborated or not we cannot yet say; but it is a well-substantiated fact that some minerals do gain in color and hardness on exposure, and equally well proved that many others lose their tints very perceptibly. Strange to say, the cystine calculi undergo a similar change of color, and assume a fine greenish-blue tint when exposed to the light, changing from a fawn color. The specimens in Guy’s Museum described by Dr. Marcet in 1817 were of a pale brown, but according to the report of Golding Bird they now resemble the green sulphate of iron. Dr. Peter observed the same mysterious change of color in the two cystine calculi preserved in the museum of Transylvania University, and noted the fact that the change takes place on the side exposed to the light. But one locality thus far has been discovered in the United States or even in North America, and this occurs in North Carolina. For several years previous to the year 1880, Mr. J. A. Stephenson, a collector of minerals, had obtained in Alexander County a number of beryls and crystals of transparent minerals which had the shape of beryl with a tint of the emerald hue, also other crystals of acicular form which exhibited the true color of the finest Granada specimens of emeralds.