AGATE, or Achat (among the Greeks and Latins Aχαrjjς and Achates, 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 neither formed by repeated incrustations round a central nucleus, nor 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 dendrachtes, mocoa stone, or arborescent agate. 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 milhy-loohing 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 phassachates 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 hoemachates, or the bloody agate, by the ancients. 3. The clouded and spotted agate, of a pale flesh colour, called by the ancients the carnelian achates or sardachates. 4. The red-lead coloured one, variegated with yellow, called the coral agate, or coralla 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 leonteseres, by the ancients.

Lastly, Of the agates with a greenish ground, there is only one known species, called by the ancients jaspachates.

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. Those 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 aphrodisian agate. An agate is mentioned by Kircher,^[1. Ephem. German. dec. i∙ an. 1. obs. 151. ] 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,^[2. De Gem. 1. ü. c. 95. ] 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.^[3. Pliny. 1. xxxvii∙ c. 3. ] We have also seen accounts of an oriental agate, of such size as to be fashioned into a cup, whose diameter is an ell abating two inches. In the cavity is found delineated in black specks, b. xristor. s. 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 anthropomorphous.

Agates may be stained artificially with solution of silver in spirit of nitre, and afterwards exposing the part to the [7:2:242]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 these 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.

The agate is used for making cups, rings, seals, handles for knives and forks, hilts for swords and hangers, beads to pray with, smelling-boxes, patch-boxes, &c. being cut or sawed with no great difficulty.

Agate, 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.

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.