or Achat (among the Greeks and Latins Agate, and Achates, from a river in Sicily, on the banks of which it was first found), a name applied by mineralogists to a siliceous stone of the quartz family, generally occurring in rounded nodules, or in veins in trap rocks. The number of agate balls in the rock often give it the character of amygdaloid; and when such a rock decomposes by the elements, the agates drop out, and are found in the beds of streams descending from such mountains; or they may be obtained in quarrying. Immense quantities are obtained from Oberstein and Idar, in Germany, and many are brought from India and Brazil. Very large masses of chalcedony, a variety of agate, are brought from Iceland, Feroe, and Brazil, which often have a mammillated surface, and are very uniform in colour. A large quantity of agate is found in Scotland; whence the stone is familiarly known to our lapidaries as Scotch Pebble. Agate chiefly consists of chalcedony, with mixtures of common quartz, and occasional patches of jasper and opal. The colour delineations are often in concentric rings of varying forms and intensity, or in straight parallel layers or bands. The colours are chiefly gray, white, yellow, or brownish-red. The composition of agate is not uniform; but it usually contains from 70 to 96 per cent. of silica, with varying proportions of alumina, coloured by oxide of iron or manganese. The principal varieties are—

1. Chalcedony. In this the colours are in parallel bands. Notwithstanding the compact structure of agate, it is now known to be porous, though the eye cannot detect the cavities; and this has given rise to a beautiful and important process for heightening the natural colours of the stone artificially. This has been long secretly practised at Oberstein and Idar, the seats of the great agate manufactories of Europe, and probably has been long known in India, especially for the production of the finely coloured Carnelian and Mochas of that country. The stones best suited for this purpose are such as when recently fractured most readily imbibe moisture from the tongue; and a rude guess is made of the value of the specimen, by the quickness with which the moisture disappears.

The stone is first dried without heat, and is then immersed in a mixture of a quarter of a pound of honey in a pint of water; the whole is placed in an oven heated below the boiling point, where it should remain for two or three weeks, constantly covered with the liquid. At the end of this time the stones are washed, dried, and introduced into an earthenware vessel, containing sufficient sulphuric acid to cover the stones; this vessel, covered with a lid, is next placed in the oven, for a space varying from one to twelve hours, according to the hardness of the stone. The agates are now removed from the vessel, washed, and thoroughly dried; after which they are kept in oil for twenty-four hours; the oil is removed by rubbing them with bran. The stones are now cut and polished. In the best specimens, the gray streaks are increased in intensity; some exhibit brown streaks approaching to black, while white impenetrable parts assume a brighter hue by the contrast. This is the process employed to convert the veined chalcedony or agate into onyx, for the purposes of the lapidary, especially in the production of cameos and intaglios, in imitation of the antique sculptured gems; of which most admirable specimens have descended to us, and are found in the cabinets of the curious, especially in the Florentine Museum. In those minute but exquisite works, the ancient Greeks especially excelled; and curious specimens of the art are still found among the tombs of Egypt, Assyria, and Etruria. Among the moderns the Romans are the most successful imitators of antiquity; and the name of Pickler was long mentioned as almost a rival of the ancient engravers of gems. In such works the figures, whether in relief or intaglio, appear of a different colour from the ground.

A beautiful clear yellow is given to agate by digestion in hydrochloric acid for two or three weeks, at a moderate heat. The stone is first dried for two days in an oven, and immersed, when hot, in the acid; the jar is lined with clay, and placed in an oven for the requisite time. When examined, the muddy-brown streaks will be found of a rich yellow. The change that takes place is probably on the oxide of iron. Chalcedony has also been coloured, so as to imitate the Turquoise; but the process is not divulged. It is perhaps effected by immersing it in a solution of copper, which is the colouring matter of Turquoise.

2. Carnelian, or red chalcedony, when found, is almost always brownish or muddy. Both this sort and the yellowish-brown varieties are converted into a rich red by roasting, so as to rival the Indian carnelian, which probably also has its colour heightened artificially. The following process is employed at Oberstein, when the pale red stone becomes of a bright full red, and the muddy yellow of a rich full carnelian hue. Such stones are first kept in an oven for two or three weeks to dissipate moisture; they are then dipped in sulphuric acid, and immediately exposed in a covered earthenware crucible to a red heat; the whole is allowed to cool slowly; the stones are removed when cold, and washed. The hydroxide of iron they naturally contained has lost its water, and is more highly oxidised, and thus the full colour is produced.

3. Mocha stones, originally brought from the East, are clear grayish chalcedones, with clouds and dashes of rich brown of various shades. They probably owe their colour chiefly to art. 4. Moss-agates are such as contain arborizations, or dendrites of oxide of iron; these seem in some instances to be produced on real vegetable forms, as petrifactions; but some of them are imitative forms that oxidize of iron and manganese are known to assume.

5. Jasper-agate and Opal-agate, are mixtures of agate with these minerals.

6. Plasma, a substance found in engraved stones in the ruins of Rome, also on the Schwarzwald near Baden, and on Mount Olympus, appears to be chalcedony, coloured by oxide of iron, as it occurs in green-earth.

7. Chrysoprase is now considered as a quartz, or agate, coloured by oxide of nickel.

Agates have been described with representations of men, animals, or inanimate natural objects; but we can now have no hesitation in considering them as productions of art, or ingenious deceptions. Velschius had in his custody a flesh-coloured agate, on one side of which appeared a half-moon in great perfection, represented by a milky semicircle; on the other side, the phases of reaper, or the evening star; whence he denominated it an aphroditean agate. An agate is mentioned by Kircher, on which was the representation of a heroic armed; and one in the church of St Mark in Venice has the representation of a king's head adorned with a diadem. On another, in the museum of the Prince of Gonzaga, was represented the body of a man with all his clothes, in a running attitude. A still more curious one is mentioned by De Boot, wherein appears a circle struck in brown, as exactly as if done with a pair of compasses, and in the middle of the circle the exact figure of a bishop with a mitre on; but inverting the stone a little, another figure appears; and if it is turned yet farther, two others appear, the one of a man, and the other of a woman. But the most celebrated agate of this kind was that of Pyrrhus, wherein were represented the nine Muses, with their proper attributes, and Apollo in the middle playing on the harp. We have also seen accounts of an oriental agate, of such size as to be fashioned into a cup, with a diameter of an ell abating two inches. In the cavity is found delineated in black specks, n. xiii.ior. & xxx. Other agates have also been found, representing the numbers 4191, 191; whence they were called arithmetical agates, as those representing men or women have obtained the name of anthropomorphic.

The agate is used for making cups, rings, seals, handles for knives and forks, hilts for swords and hangers, rosary beads, and a great variety of trinkets; being cut or sawed with no great difficulty.

Agate, among Antiquaries, denotes a stone of this kind engraven by art.

Agate is also the name of an instrument used by gold-wire drawers; so called from the agate in the middle of it, which forms its principal part.