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Rh rock is viscous, and thus form a large cavity which may become the home of an agate of exceptional size; thus a Brazilian geode, lined with amethyst, of the weight of 35 tons, was exhibited at the Düsseldorf Exhibition of 1902.

The first deposit on the wall of a cavity, forming the “skin” of the agate, is generally a dark greenish mineral substance, like celadonite, delessite or “green earth,” which are hydrous silicates rich in iron, derived probably from the decomposition of the augite in the mother-rock., This green silicate may give rise by alteration to a brown oxide of iron (limonite), producing a rusty appearance on the outside of the agate-nodule. The outer surface of an agate, freed from its matrix, is often pitted and rough, apparently in consequence of the removal of the original coating. The first layer spread over the wall of the cavity has been called the “priming,” and upon this basis zeolitic minerals may be deposited, as was pointed out by Dr M. F. Heddle. Chalcedony is generally one of the earlier deposits and crystallized quartz one of later formation. Tubular channels, usually choked with siliceous deposits, are often visible in sections of agate, and were formerly regarded, especially by L. von Buch and J. Nöggerath, as inlets of infiltration, by which the siliceous solutions gained access to the interior of the amygdaloidal cavity. It seems likely, however, that the solution transuded through the walls generally, penetrating the chalcedonic layers, as Heddle maintained, by osmotic action. Much of the chalcedony in an agate is known, from the method of artificially staining the stone, to be readily permeable. It was argued by E. Reusch that the cavities were alternately filled and emptied by means of intermittent hot springs carrying silica; while G. Lange, of Idar, suggested that the tension of the confined steam might pierce an outlet through some weak point in the coating of gelatinous silica, deposited on the walls, so that the tubes would be channels of egress rather than of ingress—a view supported by Heddle, who described them as “tubes of escape.”

It sometimes happens that horizontal deposits, or strata usually opaline in character, are formed on the floor of a cavity after the walls have been lined with successive layers of chalcedony. Many agates are hollow, since deposition has not proceeded far enough to fill the cavity, and in such cases the last deposit commonly consists of quartz, often amethystine, having the apices of the crystals directed towards the free space, so as to form a crystal-lined cavity or geode.

When the deposits in an agate have been formed on a crop of crystals, or on a rugose base, the cross-section presents a zigzag pattern, rather like the plan of a fortress with salient and retiring angles, whence the stone is termed fortification agate. If the section shows concentric circles, due either to stalactitic growth or to deposition in the form of bosses and beads on the floor, the stone is known as ring agate or eye agate. A Mexican agate, showing only a single eye, has received the name of “cyclops.” Included matter of a green colour, like fragments of “green earth,” embedded in the chalcedony and disposed in filaments and other forms suggestive of vegetable growth, gives rise to moss agate. These inorganic enclosures in the agate have been sometimes described, even after microscopic examination, as true vegetable structures. Dendritic markings of black or brown colour, due to infiltration of oxides of manganese and iron, produce the variety of agate known as Mocha stone. Agates of exceptional beauty often pass in trade under the name of Oriental agate. Certain stones, when examined in thin sections by transmitted light, show a diffraction spectrum, due to the extreme delicacy of the successive bands, whence they are termed rainbow agates.

On the disintegration of the matrix in which the agates are embedded, they are set free, and, being by their siliceous nature extremely resistant to the action of air and water, remain as nodules in the soil and gravel, or become rolled as pebbles in the streams. Such is the origin of the “Scotch pebbles,” used as ornamental stones. They are agates derived from the andesitic lavas of Old Red Sandstone age, chiefly in the Ochils and the Sidlaws. In like manner, the South American agates, so largely cut and polished at the present time, are found mostly as boulders in the beds of rivers.

An enormous trade in agate-working is carried on in a small district in Germany, around Oberstein on the Nahe, a tributary of the Rhine at Bingen. Here the industry was located many centuries ago, in consequence of the abundant occurrence of agates in the amygdaloidal melaphyre of the district, notably in the Galgenberg, or Steinkaulenberg, overlooking the village of Idar, on the Idar Bach, about two miles from Oberstein. The abundant water-power in the neighbourhood had also a share in the determination of the industrial site. At the present time, however, steam power and even electricity are employed in the mills of the Oberstein district. Although the agate industry is still carried on there, especially at Idar, the stones operated on are not of indigenous origin, but are imported mostly from Brazil (Rio Grande do Sul) and from Uruguay, where they were discovered in 1827. Agate-working is also carried on to a limited extent at Waldkirch in the Black Forest.

Most commercial agate is artificially stained, so that stones naturally unattractive by their dull grey tints come to be valuable for ornamental purposes. The art of staining the stone is believed to be very ancient. Possibly referred to by Pliny (bk. xxxvii. cap. 75), it was certainly practised at an early date by the Italian cameo-workers, and from Italy a knowledge of the art—long kept secret and practised traditionally—passed in the early part of the 19th century to the agate-workers in Germany, by whom it has since been greatly developed. The colouring matter is absorbed by the porosity of the stone, but different stones and even different layers in the same stone exhibit great variation in absorptive power. The Brazilian agates lend themselves readily to coloration, while the German agates are much less receptive.

To produce a dark brown or black colour, the stone is kept perhaps for two or three weeks in a saccharine solution, or in olive oil, at a moderate temperature. After removal from this medium, the agate is well washed and then digested for a short time in sulphuric acid, which entering the pores chars or carbonizes the absorbed sugar or oil. Certain layers of chalcedony are practically impermeable, and these consequently remain uncoloured, so that an alternation of dark and white bands is obtained, thus giving rise to an onyx. If stained too dark, the colour may be “drawn,” or lightened, by the action of nitric acid.

Agate is stained red, so as to form carnelian and sardonyx, by means of ferric oxide. This may be derived from any iron compound naturally present in the stone, especially from limonite by dehydration on baking. Some stones are “burnt” by mere exposure to the heat of the sun, whereby the brown colour passes to red. Usually, however, an iron-salt, like ferrous sulphate, is artificially introduced in solution and then decomposed by heat, so as to form in the pores a rich red pigment.

A blue colour, supposed to render the agate rather like lapis lazuli, is produced by using first an iron salt and then a solution of ferrocyanide or ferricyanide of potassium; a green colour, like that of chrysoprase, is obtained by means of salts of nickel or of chromium; and a yellow tint is developed by the action of hydrochloric acid.

Among the uses to which agate is applied may be mentioned the formation of knife-edges of delicate balances, small mortars and pestles for chemical work, burnishers and writing styles, umbrella-handles, paper-knives, seals, brooches and other trivial ornaments. Most of these are cut and polished in the Oberstein district, at a very cheap rate, from South American stones.

Numerous localities in the United States and Canada yield agates, as described by Dr G. F. Kunz. They are abundant in the trap rocks of the Lake Superior region, some of the finest coming from Michipicoten Island, Ontario. A locality on the shore of the lake is called Agate Bay. Wood agate, or agatized wood, is not infrequently found in Colorado, California and elsewhere in the West, the most notable locality being the famous “silicified” forest known as Chalcedony Park, in Apache county,