Of all the emerald imitations that have descended to us from antiquity, none are more remarkable, none more interesting to the antiquary and historian, than the famous Sacro Catino of the cathedral of Genoa. This celebrated relic is a glass dish or patera fourteen inches in width, five inches in depth and of the richest transparent green color, though disfigured by several flaws. It was bestowed upon the republic of Genoa by the Crusaders after the capture of Caesarea in 1101, and was regarded as an equivalent for a large sum of money due from the Christian army. It was traditionally believed to have been presented to King Solomon by the queen of Sheba, and afterward preserved in the Temple, and some accounts relate that it was used by Christ at the institution of the Lord's Supper. The Genoese received it with so much veneration and faith that twelve nobles were appointed to guard it, and it was exhibited but once a year, when a priest held it up in his hand to the view of the passing throng. The state in 1319, in a time of pressing need, pawned the holy relic for twelve hundred marks of gold (two hundred thousand dollars), and redeemed it with a promptness which proved its belief in the reality of the material as well as in its sanctity. And it is also related that the Jews, during a period of fifty years, lent the republic four million francs, holding the sacred relic as a pledge of security. Seven hundred years passed away, when Napoleon came, and as he swept down over Italy, gathering her art-treasures, he ordered the "Holy Grail" to be conveyed to Paris. It was deposited in the Cabinet of Antiquities in the Imperial Library, and the mineralogists quickly discovered it to be glass. It is due to the memory of Condamine to state that he was the first to doubt the material of the Sacro Catino, for, when examining it by lamplight in 1757, in the presence of the princes Corsini, he observed none of the cracks, clouds and specks common to emeralds, but detected little bubbles of air. In 1815 the Allies ordered its return to the cathedral of Genoa. During this journey the beautiful relic was broken, but its fragments were restored by a skillful artisan, and it is now supported upon a tripod, the fragments being held together by a band of gold filigree. This remarkable object of antiquity, which is of extraordinary beauty of material and workmanship, furnishes a theme over which the antiquaries love to muse and wrangle.
Another of the antique monster emeralds, weighing twenty-nine pounds, was presented to the abbey of Reichenau near Constance by Charlemagne. Beckman has also detected this precious relic to be glass. And probably the great emerald of two pounds weight brought home from the Holy Land by one of the dukes of Austria, and now deposited in the collection at Vienna, is of the same material. The hardness of our glass is yet far inferior to that of the ancients, and even the ruby lustre of the potters of Umbria, which was so precious to the dilettanti of the Cinque Cento period, has not been recovered.
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. 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.[2]
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 jeweler. 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 steatite of St. 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 dessication, 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.
Since the time of the Spanish Conquest, New Granada has furnished the world with the most of its emeralds. The most famous mines are at Muzo, in the valley of Tunca, between the mountains of New Granada and Popayan, about seventy-five miles from Santa Fé de Bogota, where every rock, it is said, contains an emerald. At present the supply of emeralds is very limited, owing to restrictions on trade and want of capital and energy in mining operations.
Blue as well as green emeralds are found in the Cordillera of the Cubillari. The Esmeraldas mines in Equador are said to have been worked successfully at one period by the Jesuits. The Peruvians obtained many emeralds from the barren district of Atacama, and in the times of the Conquest there were quarries on the River of Emeralds near Barbacoas.