or Achat, (among the Greeks and Latins, ἀχάτης, and ἀχάτης, from a river in Sicily, on the banks of which it was first found), a very extensive genus of the semipellucid gems.

These stones are variegated with veins and clouds, but have no zones like those of the onyx. They are composed of crystal debased by a large quantity of earth, and not formed, either by repeated incrustations round a central nucleus, or made up of plates laid evenly on one another; but are merely the effect of one simple concretion, and variegated only by the disposition given, by the fluid they were formed in, to their differently coloured veins or matters.

Agates are arranged according to the different colours of their ground. Of those with a white ground there are three species. (1.) The dendrachates*, mocco stone, or arborecent agat. This seems to be the same with what some authors call the achates with rosemary in the middle, and others achates with little branches of black leaves. (2.) The dull milky-looking agate. This, though greatly inferior to the former, is yet a very beautiful stone. It is common on the shores of rivers in the East Indies, and also in Germany and some other parts of Europe. Our lapidaries cut it into counters for card-playing, and other toys of small value. (3.) The lead-coloured agate, called the phylla-chates† by the ancients.

Of the agates with a reddish ground there are four species. (1.) An impure one of a flesh-coloured white, which is but of little beauty in comparison with other agates. The admixture of flesh-colour is but very slight; and it is often found without any clouds, veins, or other variegations; but sometimes it is prettily veined or variegated with spots of irregular figures, having fimbriated edges. It is found in Germany, Italy, and some other parts of Europe; and is wrought into toys of small value, and often into the German gunflints. It has been sometimes found with evident specimens of the perfect mosses bedded deep in it. (2.) That of a pure blood colour, called hemachates‡, or the bloody agate, by the ancients. (3.) The clouded and spotted agate, of a pale flesh colour, called by the ancients the cornelian agate, or jardachates§. (4.) The red-lead-coloured one, variegated with yellow, called the coral agate, or corallo-achates$, by the ancients.

Of the agates with a yellowish ground there are only two known species: the one of the colour of yellow wax, called cerachates by the ancients; the other a very elegant stone, of a yellow ground, variegated with white, black, and green, called the leonina, and leontesferes†, by the ancients.

Lastly, Of the agates with a greenish ground, there is only one known species, called by the ancients jas-pachates‡.

Of all these species there are a great many varieties; some of them having upon them natural representations of men and different kinds of animals, &c. These representations are not confined to the agates whose ground is of any particular colour, but are occasionally found on all the different species. 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 vesper, or the evening-star; whence he denominated it an aphroditean agate. An agate is mentioned by Kircher*, on which was the representation of a heroine 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 posture. 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 further, two others appear, the one of a man, and the other of a woman. But the most celebrated agate of this kind is that of Pyrrhus, wherein were represented the nine muses, each with their proper attributes, and Apollo in the middle playing on the harp. In the emperor's cabinet is an oriental agate of a surprising bigness, being fashioned into a cup, whose diameter is an ell, abating two inches. In the cavity is found delineated in black specks, ΘΕΡΙΣΤΟΡΟΣ. 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 anthropomorphous.

Great medicinal virtues were formerly attributed to the agate, such as resisting poisons, especially those of the viper, scorpion, and spider; but they are now very justly rejected from medicinal practice. The oriental ones are all said to be brought from the river Gambay. A mine of agates was some time ago discovered in Transylvania, of divers colours; and some of a large size, weighing several pounds.

Agates may be stained artificially with solution of silver in spirit of nitre, and afterwards exposing the place to the sun‡; and though these artificial colours disappear on laying the stone for a night in aquafortis, yet a knowledge of the practicability of thus staining agates, must render those curious figures above-mentioned strongly suspected of being the work not of nature, but of art. Some account for these phenomena from natural causes. Thus, Kircher, who had seen a stone of this kind in which were depicted the four letters usually inscribed on crucifixes, I. N. R. I., apprehends that some real crucifix had been buried underground, among stones and other rubbish, where the inscription happening to be parted from the cross, and to be received among a soft mould or clay susceptible... of the impression of the letters, came afterwards to be petrified. In the same manner he supposes the agate of Pyrrhus to have been formed. Others resolve much of the wonder into fancy, and suppose those stones formed in the same manner with the camiceux* or Florentine stones.

The agate is used for making cups, rings, seals, handles for knives and forks, hilts for swords and hangers, heads to pray with, smelling-boxes, patch-boxes, &c., being cut or sawed with no great difficulty. At Paris, none have a right to deal in this commodity except the wholesale mercers and goldsmiths. The sword-cutters are allowed to sell it, but only when made into handles for conteneux de chaffe, and ready set in. The cutlers have the same privilege for their knives and forks.

Considerable quantities of these stones are still found near the river Achates in Sicily. There are found in some of these the surprising representations above-mentioned, or others similar to them. By a dextrous management of these natural stains, medals have been produced, which seem master-pieces of nature; for this stone bears the graver well; and as pieces of all magnitudes are found of it, they make all sorts of work it. The high altar of the cathedral of Messina is all over encrusted with it. The lapidaries pretend that the Indian agates are finer than the Sicilian; but father Labat* informs us, that in the same quarries, and even in the same block, there are found pieces much finer than others, and these fine pieces are sold for Indian agates in order to enhance their price.

among antiquaries, denotes a stone of this kind engraven by art. In this sense, agates make a species of antique gems, in the workmanship whereof we find eminent proofs of the great skill and dexterity of the sculptors. Several agates of exquisite beauty are preserved in the cabinets of the curious; but the facts or histories represented on these antique agates, however well executed, are now become so obscure, and their explications so difficult, that several diverting mistakes and disputes have arisen among those who undertook to give their true meaning.

The great agate of the apotheosis of Augustus, in the treasury of the holy chapel, when sent from Constantinople to St Lewis, passed for a triumph of Joseph. An agate now in the French king's cabinet, had been kept 700 years with great devotion, in the Benedictine abbey of St Evre at Toul, where it passed for St John the evangelist carried away by an eagle, and crowned by an angel; but the heathenism of it having been lately detected, the religious would no longer give it a place among their relics, but presented it in 1684 to the king. The antiquaries found it to be the apotheosis of Germanicus. In like manner the triumph of Joseph was found to be a representation of Germanicus and Agrippina, under the figures of Ceres and Triptolemus. Another was preserved, from time immemorial, in one of the most ancient churches of France, where it had passed for a representation of paradise and the fall of man; there being found on it two figures representing Adam and Eve, with a tree, a serpent, and an Hebrew inscription round it, taken from the third chapter of Genesis, "The woman saw that the tree was good, &c." The French academists, instead of our first parents, found Jupiter and Minerva represented by the two figures: the inscription was of a modern date, written in a Rabbinical character, very incorrect, and poorly engraven. The prevailing opinion was, that this agate represented simply the worship of Jupiter and Minerva at Athens.

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