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AGATE

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Originally appearing in Volume V01, Page 370 of the 1911 Encyclopedia Britannica.
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AGATE , a See also:

term applied not to a distinct See also:mineral See also:species, but to an aggregate of various forms of See also:silica, chiefly See also:Chalcedony (q.v.). According to See also:Theophrastus the agate (&Xartls) was named from the See also:river See also:Achates, now the Drillo, in See also:Sicily, where the See also:stone was originally found. Most agates occur as nodules in eruptive rocks, or See also:ancient lavas, where they represent cavities originally produced by the disengagement of vapour in the molten See also:mass, and since filled, wholly or partially, by siliceous See also:matter deposited in See also:regular layers upon the walls. Such agates, when cut transversely, exhibit a See also:succession of parallel lines, often of extreme tenuity, giving a banded See also:appearance to the See also:section, whence such stones are known as banded agate, riband agate and striped agate. Certain agates also occur, to a limited extent, in See also:veins, of which a notable example is the beautiful brecciated agate of Schlottwitz, near Wesenstein in Saxony—a stone mostly composed of angular fragments of agate cemented with amethystine See also:quartz. In the formation of an See also:ordinary agate, it is probable that See also:waters containing silica in solution—derived, perhaps, from the decomposition of some of the silicates in the See also:lava itself—percolated through the See also:rock, and deposited a siliceous coating on, the interior of the vapour-vesicles. See also:Variations in the See also:character of the See also:solution, or in the conditions of See also:deposit, may have caused corresponding variation in the successive layers, so that bands of chalcedony often alternate with layers of crystalline quartz, and occasionally of opaline silica. By See also:movement of the lava, when originally viscous, the vesicles were in many cases See also:drawn out and compressed, whence the mineral matter with which they became filled assumed an elongated See also:form, having the longer See also:axis in the direction in which the magma flowed. From the fact that these kernels are more or less See also:almond-shaped they are called amygdales, whilst the rock which encloses them is known as an See also:amygdaloid. ,,Several vapour-vesicles may unite while the rock is viscous, and thus form a large cavity which may become the See also:home of an agate of exceptional See also:size; thus a Brazilian geode, lined with See also:amethyst, of the See also:weight of 35 tons, was exhibited at the See also:Dusseldorf See also:Exhibition of 1902. The first deposit on the See also:wall of a cavity, forming the " skin " of the agate, is generally a dark greenish mineral substance, like celadonite, delessite or " See also:green See also:earth," which are hydrous silicates See also:rich in See also:iron, derived probably from the decomposition of the See also:augite in the See also:mother-rock. ' This green silicate may give rise by alteration to a See also:brown See also:oxide of iron (See also:limonite), producing a rusty appearance on the outside of the agate-nodule.

The See also:

outer See also:surface of an agate, freed from its See also:matrix: is often pitted and rough, apparently in consequence of the removal of the See also: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. See also: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 See also:Buch and J. Noggerath, as inlets of infiltration, by which the siliceous solutions gained See also:access to the interior of the amygdaloidal cavity. It seems likely, however, that the solution transuded through tha walls generally, penetrating the chalcedonic layers, as Heddle maintained, by osmotic See also: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. See also:Reusch that the cavities were alternately filled and emptied by means of intermittent hot springs carrying silica; while G. See also:Lange, of See also:Idar, suggested that the tension of the confined See also:steam might See also: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 See also:egress rather than of ingress—a view supported by Heddle, who described them as " tubes of See also:escape." It sometimes happens that See also:horizontal deposits, or strata usually opaline in character, are formed on the See also: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 See also:

free space, so as to form a crystal-lined cavity or geode. When the deposits in an agate have been formed on a See also:crop of crystals, or on a rugose See also:base, the See also:cross-section presents a zigzag See also:pattern, rather like the See also: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 See also:ring agate or See also:eye agate. A Mexican agate, showing only a single eye, has received the name of " cyclops." Included matter of a green See also:colour, like fragments of " green earth," embedded in the chalcedony and disposed in filaments and other forms suggestive of See also:vegetable growth, gives rise to See also:moss agate. These inorganic enclosures in the agate have been sometimes described, even after microscopic examination, as true vegetable structures. Dendritic markings of See also:black or brown colour, due to infiltration of oxides of See also:manganese and iron, produce the variety of agate known as Mocha stone. Agates of exceptional beauty often pass in See also:trade under the name of See also:Oriental agate. Certain stones, when examined in thin sections by transmitted See also:light, show a diffraction spectrum, due to the extreme delicacy of the successive bands, whence they are termed See also:rainbow agates. On the disintegration of the matrix in which the agates are embedded, they are set free, and, being by their siliceous nature eslremely resistant to the action of See also:air and See also:water, remain as n±tles in the See also:soil and See also:gravel, or become rolled as pebbles in the streams. Such is the origin of the " Scotch pebbles," used Qs ornamental stones. They are agates derived from the andesitic See also:J1 was of Old Red See also:Sandstone See also:age, chiefly in the Ochils and the 5id4xws. In like manner, the See also:South See also:American agates, so largelycut and polished at the See also:present See also:time, are found mostly as boulders in the beds of See also:rivers.

An enormous trade in agate-working is carried on in a small See also:

district in See also:Germany, around See also:Oberstein on the See also:Nahe, a tributary of the See also:Rhine at See also:Bingen. Here the See also: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 See also:village of Idar, on the Idar See also:Bach, about two See also:miles from Oberstein. The abundant water-See also:power in the neighbourhood had also a See also:share in the determination of the See also:industrial site. At the present time, however, steam power and even See also:electricity are employed in the See also: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 See also:Brazil (Rio Grande do Sul) and from See also:Uruguay, where they were discovered in 1827. Agate-working is also carried on to a limited extent at Waldkirch in the Black See also:Forest. Most commercial agate is artificially stained, so that stones naturally unattractive by their dull See also:grey tints come to be valuable for ornamental purposes. The See also:art of staining the stone is believed to be very ancient. Possibly referred to by See also:Pliny (bk. See also:xxxvii. cap. 75), it was certainly practised at an See also:early date by the See also:Italian See also:cameo-workers, and from See also:Italy a knowledge of the art—long kept See also:secret and practised traditionally—passed in the early See also:part of the 19th See also:century to the agate-workers in Germany, by whom it has since been greatly See also:developed. The colouring matter is absorbed by the porosity of the stone, but different stones and even different layers in the same stone exhibit See also:great variation in absorptive power. The Brazilian agates lend them-selves readily to coloration, while the See also:German agates are much less receptive.

To produce a dark brown or black colour, the stone is kept perhaps for two or three See also:

weeks in a saccharine solution, or in See also:olive oil, at a moderate temperature. After removal from this See also:medium, the agate is well washed and then digested for a See also:short time in sulphuric See also:acid, which entering the pores chars or carbonizes the absorbed See also:sugar or oil. Certain 'layers of chalcedony are practically impermeable, and these consequently remain uncoloured, so that an See also:alternation of dark and See also:white bands is obtained, thus giving rise to an See also: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 See also:carnelian and See also:sardonyx, by means of ferric oxide. This may be derived from any iron See also:compound naturally present in the stone, especially from limonite by dehydration on See also:baking. Some stones are " burnt " by See also:mere exposure to the See also:heat of the See also:sun, whereby the brown colour passes to red. Usually, however, an iron-See also: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 See also: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 ferricyauide of See also:potassium; a green colour, like that of See also:chrysoprase, is obtained by means of salts of See also:nickel or of See also: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 See also:knife-edges of delicate balances, small mortars and pestles for chemical See also:work, burnishers and See also:writing styles, See also:umbrella-handles, See also:paper-knives, See also:seals, brooches and other trivial ornaments. Most of these are cut and polished in the Oberstein district, at a very cheap See also:rate, from South American stones. Numerous localities in the See also:United States and See also:Canada yield agates, as described by Dr G.

F. Kunz. They are abundant in the See also:

trap rocks of the See also:Lake See also:Superior region, some of the finest coming from Michipicoten See also:Island, See also:Ontario. A locality on the See also:shore of the lake is called Agate See also:Bay. See also:Wood agate, or agatized wood, is not infrequently found in See also:Colorado, See also:California and elsewhere in the See also:West, the most notable locality being the famous "silicified forest" known as Chalcedony See also:Park, in See also:Apache See also:county, See also:Arizona. Here there are vast See also:numbers of water-rolled logs of silicified wood, in rocks of Triassic age, but only a small quantity of the wood is See also:fine enough for ornamental purposes. The cellular See also:tissue of the vegetable matter is filled, or even replaced, by various siliceous minerals like chalcedony, See also:jasper, crystalline quartz and semi-See also:opal, the silica having probably been introduced by thermal waters. Some of the agate shows the microscopic structure of araucarian wood. The agatized wood is sometimes known by the See also:Indian name of shinarump. In See also:India agates occur abundantly in the amygdaloidal varieties of the See also:Deccan and See also:Rajmahal traps, and as pebbles in the detritus derived from these rocks. Some of the finest are found in the agate-gravels near Ratanpur, in See also:Rajpipla. The trade in agates has been carried on from early times at See also:Cambay, where the stones are cut and polished.

Agates are also worked at See also:

Jubbulpore. In many parts of New South See also:Wales, agates, resulting from the disintegration of trap rocks, are See also:common in the river-beds and old drifts. They occur also in See also:Queensland, as at Agate See also:Creek, See also:running into the See also:Gilbert river. South See also:Africa likewise yields numerous agates, especially in the gravels of the See also:Orange and See also:Vaal rivers. It should be noted that in See also:England agates are found not only in old lavas, like the andesites of the Cheviots, but also to a limited extent in the Dolomitic See also:Conglomerate, an old See also:beach-deposit of Triassic age in the Mendips and the 'neighbourhood of See also:Bristol. They are also found as weathered pebbles in the See also:drift of See also:Lichfield in See also:Staffordshire. For Scottish agates see M. F. Heddle, " On the Structure of Agates," Trans. Geolog. See also:Soc. See also:Glasgow, vol. xi. part ii., 1900, p.

153; and See also:

Mineralogy of See also:Scotland (1901), vol. i. p. 58; J. G. See also:Good-See also:child, Proc. Phys. Soc. See also:Edinburgh, vol. xiv., 1899, p. 191. For the agate-industry see G. Lange, See also:Die Halbedelsteine (See also:Kreuznach, 1868). For American agates, G. F.

. Kunz, Gems and See also:

Precious Stones of See also:North See also:America (189o), p. 128. For agates in See also:general see Max See also:Bauer's Precious Stones, translated by L. J. See also:Spencer (See also:London, 1904). (F. W.

End of Article: AGATE

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