Leisure hours among the gems

by Augustus C. Hamlin

One of the finest gems that adorned the gorgeous harness of Runjeet Singh was a beautiful emerald maltreated in this manner. Major Pearse found in a Punjaub tope a reliquary formed from an emerald three inches long and two inches thick, with the ends rounded off. It was originally a gem of fine color, but had been bored half through its axis to contain two finger joints of some revered Buddhist saint or petted monkey.

The emerald has been a subject of controversy among the chemists and mineralogists, and its character, especially the cause of its beautiful color, is not clearly defined even at the present day. But that distinguished chemist, Professor Lewy of Paris, seems to offer, thus far, the most correct and plausible theory. More than ten years ago he boldly asserted that the hue is not due to the oxide of chromium, and with this opinion he confronted such eminent men as Vauquelin, Klaproth, and others of high rank in the scientific world. Not content with his researches in his laboratory in Paris, he resolutely crossed the ocean and sought the emerald in its parent ledges in the lofty table-lands of New Granada. Here he obtained new information of a geological character which goes far to strengthen his position. The experiments of M. Lewy indicate, if they do not prove, that the coloring matter of the emerald is organic, and readily destroyed by heat, which would not be the case if it was due to the oxide of chromium. All my own fire-tests with the Granada emerald corroborate the views of M. Lewy, for in every instance the gem lost its hue when submitted to a red heat.

Nevertheless, the recent researches of Wöhler and Rose give negative results. These experienced chemists kept an emerald at the temperature of melted copper for an hour, and found that, although the stone had become opaque, the color was not affected. They therefore considered the oxide of chromium to be the coloring agent, without, however, denying the presence of organic matter. The amount of the oxide of chromium found by many chemists varies from one to two per cent, while Lewy and others found it in a quantity so small as to be inappreciable, and too minute to be weighed.

Before the ordinary blowpipe the emerald passes rapidly into a whitish vesicular glass, and with borax it forms a fine green glass, while its sub-species, the beryl, changes into a colorless bead; with salt of phosphorus it slowly dissolves, leaving a silicious skeleton.

M. Lewy visited the mines at Muzo in Granada, and from the results of his analyses, together with the fact of finding emeralds in conjunction with the presence of fossil shells in the limestone in which they occur, he arrived at the conclusion that they have been formed in the wet way,—deposited from a chemical solution. He also found that when extracted they are so soft and fragile that the largest and finest fragments can be reduced to powder by merely rubbing them between the fingers, and the crystals often crack and fall to pieces after being removed from the mine, apparently from loss of water. Consequently, when the emeralds are first extracted they are laid aside carefully for a few days until the water is evaporated.

This statement relative to the softness of the gem and its subsequent hardening has been met with a shout of derision from some of the gem-seekers,—none louder than that of Barbot, the retired jeweller. Barbot seems to forget that the rock of which his own house in Paris is constructed undergoes the same change after being removed from the deep quarries in the catacombs under the city.

This phenomenon is observed with many rocks. Flints acquire additional toughness by the evaporation of water contained in them. The yellow gneiss of Ceylon is soft when quarried, but hardens on exposure to the atmosphere. The Egyptian verde antique marble, which was named after Augustus and Tiberias, was easily quarried with steel implements, but quickly hardened on exposure to the external air. The mosaic plates of this mineral which are inlaid in the decorative work of the Tuscan Gothic buildings are yet quite hard. The steatite of Saint Anthony’s Falls grows harder on exposure, and other minerals, when quarried from considerable depths, become firmer on exposure to the action of the air. Observations of this kind led Kuhlman to investigate the cause; and he believes that the hardening of rocks is not owing solely to the evaporation of quarry-water, but that it depends upon the tendency which all earthy matters possess to undergo a spontaneous crystallization by slow desiccation, which commences the moment the rock is exposed to the air.

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.