Agate

AGATE, a term applied not to a distinct mineral species, but to an aggregate of various forms of silica, chiefly Chalcedony. According to Theophrastus the agate (achates) was named from the river Achates, now the Drillo, in Sicily, where the stone was originally found.  Most agates occur as nodules in eruptive rocks, or ancient lavas, where they represent cavities originally produced by the disengagement of vapor in the molten mass, and since filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls.  Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section, whence such stones are known as banded agate, riband agate and striped agate.  Certain agates also occur, to a limited extent, in 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 quartz.

In the formation of an ordinary agate, it is probable that waters containing silica in solution—derived, perhaps, from the decomposition of some of the silicates in the lava itself—percolated through the rock, and deposited a siliceous coating on the interior of the vapor-vesicles.  Variations in the character of the solution, or in the conditions of 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 movement of the lava, when originally viscous, the vesicles were in many cases drawn out and compressed, whence the mineral matter with which they became filled assumed an elongated form, having the longer axis in the direction in which the magma flowed.  From the fact that these kernels are more or less almond-shaped they are called amygdales, whilst the rock which encloses them is known as an amygdaloid.  Several vapor-vesicles may unite while the 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 Dusseldorf 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. Noggerath, 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 neighborhood 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 practiced at an early date by the Italian cameo-workers, and from Italy a knowledge of the art—long kept secret and practiced traditionally—passed in the early part of the 19^th century to the agate-workers in Germany, by whom it has since been greatly developed.  The coloring 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 color, 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 uncolored, so that an alternation of dark and white bands is obtained, thus giving rise to an onyx.  If stained too dark, the color 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 color 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 color, 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 color, 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, Arizona.  Here there are vast numbers of water-rolled logs of silicified wood, in rocks of Triassic age, but only a small quantity of the wood is fine enough for ornamental purposes.  The cellular tissue of the vegetable matter is filled, or even replaced, by various siliceous minerals like chalcedony, jasper, crystalline quartz and semi-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 Indian name of shinarump.

In India agates occur abundantly in the amygdaloidal varieties of the Deccan and 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 Rajpipla.  The trade in agates has been carried on from early times at Cambay, where the stones are cut and polished.  Agates are also worked at Jubbulpore.

In many parts of New South Wales, agates, resulting from the disintegration of trap rocks, are common in the river-beds and old drifts.  They occur also in Queensland, as at Agate Creek, running into the Gilbert river.  South Africa likewise yields numerous agates, especially in the gravels of the Orange and Vaal rivers.

It should be noted that in England agates are found not only in old lavas, like the andesites of the Cheviots, but also to a limited extent in the Dolomitic Conglomerate, an old beach deposit of Triassic age in the Mendips and the neighborhood of Bristol.  They are also found as weathered pebbles in the drift of Lichfield in Staffordshire.

For Scottish agates see M. F. Heddle, “On the Structure of Agates,” Trans.  Geolog.  Soc. Glasgow, vol. xi. part ii., 1900, p. 153; and Mineralogy of Scotland (1901), vol. i.  p. 58; J. G. Goodchild, Proc.  Phys.  Soc. Edinburgh, vol.  xiv., 1899, p. 191. For the agate-industry see G. Lange, Die Halbedelsteine (Kreuznach, 1868).  For American agates, G. F.  Kunz, Gems and Precious Stones of North America (1890), p. 128. For agates in general see Max Bauer’s Precious Stones, translated by L. J. Spencer (London, 1904).

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Source: 1911 encyclopedia.