Agate (from the river Achates, now Dirillo, ir. Sicily, near which it was found), one of the modifications in which silica presents itself nearly in a state of purity, deposited, not crystalline, but massive and. slightly transparent. Agate, onyx, chalcedony, carnelian, sard, chrysoprase, and many others are but varieties, differing only in external form and appearance from each other, of the one family, quartz. When other ingredients, as alumina or oxide of iron, are found associated with the silica, it appears that their presence is never in any fixed proportion, and is therefore regarded as accidental. Agates are distinguished from the other varieties by the veins of different shades of color which traverse the stone in parallel, concentric layers, often so thin as to number 50 or more in an inch. When these stripes alternated, an opaque band with one transparent, the stone was called onyx, from a fancied resemblance to the alternating lines upon the finger nail, from the Greek The modern distinction between agate and onyx refers to the direction in which the stone may be cut; when vertical to the stratification, so as to show stripes or bands, it is called agate; and the same stone, if cut parallel with the strata, is termed onyx, and forms the material of the true cameo. The veins of the agate are no doubt produced by successive deposition of one layer of silicious matter upon another, introduced in a sublimated or soluble form into the cavities of the rocks, where the agates are now found. These rocks are mostly amygdaloids, the cavities of which are filled by a variety of minerals. As the rock disintegrates, or wears away by the action of atmospheric agencies, the hard nodules of agate drop out, and are then found upon the surface, or, as is frequently the case, strewn along a sea beach or in the beds of mountain streams. Externally they are rough, and exhibit no appearance of their beautiful veined structure, which is exposed on breaking them, and still more perfectly after polishing. The largest nodules seldom exceed a foot in diameter. Various processes are adopted for increasing the lustre and heightening or darkening the colors of agates.
They are boiled in oil, or kept in warm honey, and then dropped into sulphuric acid. The layers being unequally porous, the absorbed carbonaceous matter becomes charred and blackened by the acid, and the white stripes, impenetrable to the oil, appear clearer and brighter by the contrast. Agates are thus made to assume the onyx character, which is desired by the lapidary for the production of cameos and intaglios, in imitation of the antique sculptured gems. In these the figures, whether in relief or intaglio, are of a different color from the ground. Dgestion for a few weeks in hydrochloric acid, kept at a moderate heat, gives a beautiful clear yellow color to the streaks that were before a dirty brown. Stones of a reddish hue are greatly improved in brilliancy of color by first thoroughly drying them for weeks in ovens, then dipping in sulphuric acid, heating to full red heat, and afterward slowly cooling. The changes that take place in both these processes are upon the oxide of iron, which is the coloring matter.
They may suggest other modes of producing other analogous effects. - Though the varieties of agate are mostly very common minerals in this country, as well as in the old world, those localities only are of interest which have long been famous for their production, and which still furnish all the agates required by commerce. The value of the stones depends upon the work put upon them, which from their extreme hardness is very laborious, and in the sculptured gems requires the greatest patience and skill. Such operations are not vet introduced into the United States, and the agates found everywhere accompanying the trap rocks meet no demand except from specimen hunters. The principal works for cutting and polishing agates are at Obcrstein, a small town not far from Mayence, in south Germany. Here are numerous water mills running the coarse stones for grinding down the surface of the agates, and the wheels of soft wood on which they are polished with the powder of tripoli, found in the neighborhood. They are made into trinkets, cups, seals, rings, handles for swords, knives and forks, and small mortars for grinding very hard substances used by chemists. - Moss agate, or Mocha stone, is grayish white or brownish yellow, with peculiar markings of dark metallic oxide, assuming varied fanciful forms.
Landscapes, trees, mosses, and animals have been traced in the lines formed by the arborescent deposit of the foreign mineral, chiefly oxides of iron or manganese. The numerous silicious springs of our western territory, at no distant date, have produced these, now generally known as Rocky mountain agates, some of which are of extraordinary beauty and wonderful resemblance, rivalling those in the museums of Europe that for ages have excited the admiration of the curious. Many of the latter, however, are such remarkable likenesses, that they must be regarded as exceedingly ingenious works of art. One in the British museum presents a likeness of the poet Chaucer; another in the church of St. Mark in Venice represents a king's head with a diadem. De Boot, in his treatise De Gem-mis, describes one which represents the figure of a bishop with his mitre, placed in the centre of a perfect circle. By turning the stone a little another figure appears, and turned still further the figures of a man and woman are seen. Pliny mentions one belonging to Pyr-rhus, in which were pictured the nine muses, with their proper attributes, and Apollo in the middle of the figure playing on the harp.
While nature has formed remarkable coincidences of resemblance, yet the credulous have been imposed upon by stone engravers, from the 15th century to our own day, by engraving a device on a soft chalcedony, with a uniform depth of cutting, and staining the stone as before described; then grinding off the surface to the depth of their cutting, leaving the stone smooth, but showing stain where the engraved lines had been. The same means were used to heighten natural marks, as well as the more legitimate mode of curving or waving the surface, so as to erase or bring out desired features. - Agate frequently occurs as a geode, or hollow nodule, explained by the theory of gas entering a silicious solution. Change of temperature and pressure account for the crystalline interior, and the various strata are evidently aggregations, usually concentric. The veins of color are sometimes polygonal, when, from resemblance to the angles of a fortress, it is called fortification agate.