agate

from Wikipedia, the free encyclopedia
Colorfully banded agate from Argentina
Gray-beige-white banded agate from Mexico

The agate is a variety of the mineral quartz , which only forms microcrystalline mineral aggregates in the form of drusen and almonds . A striking feature of agates are the predominantly brightly colored, strip-shaped deposits due to the rhythmic crystallization that become visible when cutting drusen or almond stones. However, there are also unstriped and single-colored agates.

Natural agates can come in almost any color. But mostly these are rather pale or pastel colored. However, it has been known since ancient times that agates can be colored well. Bright colors such as cobalt blue , magenta , violet and pink are therefore often an indication of artificial coloring (see the section on manipulations and imitations ).

Etymology and history

Theophrastus of Eresos gave the agate its name because it was found in large quantities in the river ἀχάτης " Achates " (now called Dirillo , Carabi or Canitello ) near the town of Acate in Sicily .

The agate was valued very early on. In ancient Egypt around 1000 BC. Chr. Seal cylinders , rings, gems and vessels made of agate. In the Bible in Exodus, 28, 17-20, the breastplate of the high priest, a plate set with precious stones, is described in detail. Agate amulets should help against lightning, storms and thirst.

At the beginning of the 17th century the agate industry had already grown to great importance, it took a further boom in the second half of the 18th century when agate goods began to be set first in silver and then in gold-plated tombac . This fausse jewelery shop was established in Oberstein and later also brought pure metal goods onto the market.

After 1813 the change in the color of the stones was discovered through firing , and in 1819 a trader from Idar-Oberstein brought home the secret of "blackening" acquired from a Roman stone cutter . Since then, agate dyeing has developed very quickly and has become one of the main reasons for the flourishing of the agate industry, which now also processed foreign stones, namely agate from Uruguay . This material came to Idar-Oberstein in 1834 and developed the agate industry in a manner not known before, especially as the onyxes, which arrived in abundance from South America, caused the art of stone cutting to flourish in Paris and Idar. Cameos were mainly made , including inlays , some of which were of high artistic value, and great business was done with them. The pieces exported to Africa were also called Negro money .

Varieties and modifications

In the course of time, due to the variety of training variants in color and shape, various proper names were coined for differentiation and classification, some of which are also used as trade names . Well-known variety names are among others

  • according to the color
Apricot agate
    • Apricot agate: Pink to flesh-colored agate from Botswana
    • Blood agate or meat agate : Single- colored orange to red or two-colored orange-white to red-white banded agate, see also carnelian
    • Onyx : black and white to brown and white ( Sardonyx )
    • Peace agate: white agate
    • Honey Agate : Honey Yellow Agate
  • according to the type of training
    • Construction agate or wood agate : "petrified" woods agate -mineralized by silica
    • Band agate : Term classified as unnecessary because agate has already been defined as a banded variety of chalcedony
    • Enhydros or water stone: agate almond with residual liquid, usually dries out soon after being removed from the bedrock
    • Fire agate : kidney or grape-shaped agate with embedded membranes made of iron oxide, which shows an opal-like play of colors when appropriately cut
    • Coral agate: The shape of the surface of the agate gives it its name
    • Layered agate: agate with multi-colored, almost parallel stripes (not to be confused with the special Uruguay stripes), which are used as raw material for layer stones .
    • Moss agate or Dendritenachat : Clear to milky white agate with dendritähnlichen storages from green to black green minerals. If the basic color is brown to beige, it is also called mocha stone in stores .
    • Pseudo agate (also polyhedral quartz or polygon agate ): Internally an agate due to the harness and the Druse but although similar in the center, has externally as opposed to Agate any almond or spherical shape, but a rectangular (polygonal) shape.
    • Rainbow agate (also iris agate ): Chalcedony / agate aggregate with twisted chalcedony fibers , which reflect the incident light in a rainbow-like iridescent way due to interference effects .
    • Tube agate : displacement morphosis of long prismatic crystals
    • Snake skin agate : light beige-colored aggregate with reticulated, white markings on the surface.
    • Scolecite agate : pseudomorphosis after scolecite
    • Rubble agate : agates broken by tectonics and cemented again by further mineral deposits.
    • Uruguay agate : Straight (horizontal) and parallel stripes

  • Construction agate

  • Fire agate

  • Blue to yellow agate with Uruguay banding

  • Snakeskin agate

  • Rubble agate

  • Pseudo agate

  • according to the random cross-section after opening agate almonds
    • Augenachat , glasses agate or circle agate : To a center concentrically or eccentrically located circles of agate bands which produce a spherical, eyes-like drawing in a sectional view.
    • Fortress agate: agate bands are similar to aerial photographs of fortress walls
    • Flame agate or cloud agate : Irregular banding, especially on the edge of agate almonds, which have a cloud-shaped appearance when cut.
    • Landscape agate : A landscape- like cross-section in agate glands or almonds, creates black, reddish and / or yellow structures, which are usually caused by penetrated, iron-containing solutions in hairline cracks.
    • Star agate : The irregularly shaped geodes are usually completely filled and often show a five-pointed shape when polished.

  • Eye agate

  • Fortress agate

  • Flame or cloud agate in brownish-white

  • Moss or dendrite agate

education

Agate almond with quartz gland

Agates are formed in rock cavities as their lining or filling: If it is (completely) filled, it is called an almond , if it is (incomplete) it is called a druse . As rock cavities primarily bubble spaces in volcanic rocks come into consideration, here the gases contained in the outflowing lava no longer penetrated to the surface of the lava flow and could not escape, but they were enclosed in the cooling lava and formed a cavity in it was filled with agate, quartz , calcite or other mineral substances in a later process . Both basic volcanic rocks, for example basalt , and acidic volcanic rocks, for example rhyolite , can be considered as volcanic rocks .

These bubble spaces in volcanic rocks are the most common places where agates can be formed, but agates are also formed in passages and crevices of various rocks and also in cavities of sediments. Also in fossils, e.g. Agates are observed in petrified trees, in silicified corals and in dinosaur bones.

Over the past 200 years, numerous mineralogists have dealt with the question of how the silicon dioxide-containing material penetrated these cavities and put forward various theories. Here it had to be clarified how z. B. in a basalt , so in a basic, low-silicate rock, such an accumulation of pure SiO 2 could take place. The originally existing thesis that the SiO 2 accumulation had already taken place in the molten rock and that drops of liquid silicon dioxide were already deposited in the glowing magma is now to be regarded as obsolete, if only because agates can also be found in rocks that have never been in a glowing state (see above). Today the unanimous opinion is that agates were formed in the so-called sedimentary-diagenetic range, i.e. at temperatures up to a maximum of 200 ° C.

Furthermore, the thesis was put forward that aqueous silicic acid solutions penetrated the rock cavities through so-called infiltration channels, deposited an agate band on the cavity wall and then left the cavity again. After numerous repetitions of this process, the cavity has gradually filled with agate. This so-called infiltration theory presupposes the dubious transport of large amounts of water in the rock and is therefore no longer recognized today.

When looking for a solution to the agate problem, the focus today is on a theory that was developed by Michael Landmesser at the Institute for Gemstone Research in Mainz. Landmesser assumes that in addition to larger cavities, there is a network of fine hairline cracks and capillaries in the rock, which is constantly filled with water and in which the silica required for accumulation in the form of monosilicic acid H 4 SiO 4 is transported by diffusion . According to Landmesser, it is not the transport of large amounts of water, but the diffusion process of the monosilicic acid that plays the decisive role in agate genesis. The monosilicic acid molecules react with one another in a continuous accumulation process, whereby the molecules combine via common oxygen atoms to form disilicic acids, tricilicic acids, tetrasilicic acids, etc., until finally colloidal polysilicic acids are formed, which are deposited on the walls of the cavity due to adhesion and in one The maturation process (especially the release of water) finally turns into solid cryptocrystalline quartz. This process takes place over long periods of time in which the external conditions such as pressure, temperature, and the presence of accompanying minerals, for example Fe 3+ compounds, change, which leads to the development of the characteristic agate banding.

Locations

Europe

The agate bowl , Vienna Treasury

Africa

North America

  • UNITED STATES:
    • Arizona: Mulligan Peak near Clifton
    • California: Mojave Desert
    • Florida: Tampa Bay
    • Minnesota: Duluth on Lake Superior
    • Montana: Dryhead Ranch / Pryor Mountains, Yellowstone River
    • Nevada: Black Rock Desert
    • New Mexico: Baker Egg Mine near Deming
    • Oregon: Priday (= Richardson) Ranch / Ochoco Mountains, Morrison Ranch
    • South Dakota: Black Hills near Fairburn , Tepee Canyon
    • Texas: Woodward Ranch at Alpine
    • Utah: Dugway Geode beds near Vernon , Hanksville , Crescent Junction near Moab
    • Washington: Ellensburg Blue Agates
    • Wyoming: Blue Forest
  • Mexico: in the state of Chihuahua : Rancho Coyamito, Rancho Gregoria and Rancho Agua Nueva near Ojo de Laguna, Rancho Los Aparejos near El Sueco, Rancho la Viñata near Ejido el Apache, Rancho Colorado near Nuevo Casas Grandes, Ejido Benito Juarez in the Sierra Santa Lucia (Crazy Lace Agates!) And in the state of Sonora

South America

Asia

Australia and New Zealand

morphology

Moss agate, 2.5 cm tall

The structure of agates usually consists of concentric or irregular layers that fill a cavity. The agate consists of different varieties of chalcedony , microcrystalline silica. The individual layers show coarser and finer structures and are often extremely thin, so that a few hundred come to 1 mm. In any case, completely amorphous (water-containing) silica is very rarely found in agates. The different color is usually due to iron and manganese compounds, but the onyxes (black and white layers), sardonyxes (brown and white layers) and carnelian (also sarders ) are usually artificially colored. Between and above the chalcedony there are mostly drusy amethyst layers . Very often the almond encloses a hollow gland space in which rock crystal , amethyst, calcite , zeolites , hematite and other minerals are formed. At Oberstein, all chalcedony layers nestle against the outer shape of the almond; in the Brazilian almonds there is usually a layer of plane-parallel horizontal layers inside . It is not uncommon for the channels through which the inner mass, or rather the liquid which it contained dissolved, to penetrate into the tonsil space, are exposed during grinding. So this is formed first, and progressing from the outside to the inside, the individual layers of aqueous solution are deposited, whereby the deposited mineral not infrequently blocked the way for the penetrating liquid and thus a cavity remained inside.

After the disintegration of the matrix in which the agates were embedded, they are free and, due to their silicon-containing nature, which is extremely resistant to corrosion by air or water, remain as nodules in the soil and in the gravel, even if they rolled in rivers become.

Agate is a very resilient mineral and shows no reaction to water or solar radiation. It is also quite well protected against dents and cracks, as it has a high degree of hardness and density.

use

As a gem

Agate as a gemstone on a Gothic cross in the Essen Cathedral Treasury

Agate and its varieties are mainly used as gem stones . Agates, which are distinguished by their coloring and drawing, have always been used to cut stones ( gems , cameos , seal rings ), but also to mortars and mortars , smoothing stones, ring stones, agraffes , bracelets, rosaries, stick buttons, knife handles , wefters and many other decorative or useful items Objects processed.

In technology

Because of its high hardness and resistance to chemicals, agate is also increasingly used in technology.

Agates are used as polishing stones for gilding in order to consolidate the applied gold leaf on the substrate and to polish it to a high gloss.

Mythology and esotericism

Agate is already described as a mystical stone in the Old Testament , where it symbolizes one of the twelve tribes of Israel in the breastplate ( Hoshen ) of the Jewish high priest .

The use of agate as a healing stone is passed down among others by Hildegard von Bingen (1098–1179) in her “Book of the Stones”. According to her, the stone, applied externally after contact with poison from spiders and snakes, should heal the painful area and remove and pull the poison away. From the " falling sickness " ridden people, epileptics should constantly an agate wear on the skin to be spared from outbreaks. Special dietary regulations using agate to prepare the cooking water should even be able to cure these diseases ("unless God does not want it"). Carried openly through the house in the shape of a cross, the agate is supposed to protect it against thieves.

Among esotericists today, agate or its sub-varieties are also used as a healing stone for various ailments such as eye agate and onyx against eye infections, carnelian against sore throats, varicose veins, sciatica and testicular diseases as well as the agate itself against water blisters on the feet.

As a zodiac stone , yellow agate is assigned to the zodiac sign Pisces , according to other sources the agate is generally assigned to the zodiac sign Virgo , Gemini or Scorpio .

As a monthly stone it is said to have been assigned to the month of May by the Romans of antiquity and according to Isidore of Seville (around 560–636) .

Manipulations and imitations

Agate slice divided into segments in natural color and colored in different colors

In line with fashionable tastes, the jewelry industry often makes use of the possibility of coloring the agate, which is based on the different nature of the individual layers of the stone, some of which are porous enough to soak up liquids, others not.

For example, most onyxes are artificially created by heating the agate in a dilute honey or sugar solution for two to three weeks and then boiling it in concentrated sulfuric acid. After it dries, it is sanded, put in oil for a day and, at the end, washed off with bran. The porous layer, in which the honey that has penetrated has been charred by the sulfuric acid, appears gray, brown or black, depending on the porosity, the impenetrable white, crystalline layer even lighter and shinier. Various chemicals can be used to create any color, as long as the agate absorbs liquids.

Before processing, the stone is often burned to change its color and then placed in sulfuric or nitric acid for a week or two. The coloring, however, is usually only carried out on the cut stones, although the color penetrates deeply into the stone mass and also appears more or less clearly on the break.

If agate is recolored in naturally occurring colors, it can only be distinguished from real agate under the microscope. Colors that do not occur in natural agate such as cobalt blue , magenta , purple or pink , on the other hand, are easy to recognize.

There are also "artificial agates", so-called agates, brightly colored glass similar to agate.

The declaration of colored agates is voluntary and not mandatory.

See also

literature

Monographs
  • Ursel Laarmann, Michael Landmesser, Maximilian Glas, Rupert Hochleitner, Rudolf Dröschel, Peter Jeckel: Agate: The precious stone from which Idar-Oberstein was created: history, origin, finds . In: Christian Weise (ed.): ExtraLapis . tape 19 . Christian Weise Verlag, 2000, ISBN 3-921656-54-0 , ISSN  0945-8492 .
  • Paul Rustemeyer, Peter Jeckel, Klaus Küsters, Franz J. Schmitter, Michael Wachtler: Agates - born from volcanoes . In: Christian Weise (ed.): ExtraLapis . tape 39 . Christian Weise Verlag, 2010, ISSN  0945-8492 .
  • Rudolf Dröschel, Hermann Bank: agate + jasper. Roots of the Idar-Oberstein gemstone region . Foundation Dt. Edelsteinmuseum, Idar-Oberstein 2004, ISBN 3-932515-42-0 .
  • Johann Zenz: agates . Bode, Salzhemmendorf-Lauenstein 2005, ISBN 3-925094-82-2 .
  • Christian Hülsen: Achates 1 . In: Paulys Realencyclopadie der classischen Antiquity Science (RE). Volume I, 1, Stuttgart 1893, column 211 (on the Achates river).
  • August Nies: Achates 2 . In: Paulys Realencyclopadie der classischen Antiquity Science (RE). Volume I, 1, Stuttgart 1893, Col. 211 (on the quartz discussed here).
Compendia
  • Petr Korbel, Milan Novák: Encyclopedia of Minerals . Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 88, 94–95 ( Dörfler Natur ).
  • Bernhard Bruder: Beautified stones. Recognizing imitations and manipulations in gemstones and minerals . Neue Erde Verlag, 2005, ISBN 3-89060-079-4 , p. 39-40 .
  • Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 148, 152 .

Web links

Commons : Agates  - Collection of images, videos and audio files
Wiktionary: agate  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Gerd Spittler: The way of the agate to the Tuareg - a journey around half the world . In: Geographical Rundschau . 54th year, no. 10 , 2002, p. 46–51 ( The way of the agate to the Tuareg ( Memento of March 4, 2016 in the Internet Archive ) [PDF]).
  2. ↑ Rudolf Rykart: Quartz monograph. The peculiarities of rock crystal, smoky quartz, amethyst and other varieties . 1st edition. Ott, Thun 1989, ISBN 3-7225-6293-7 .
  3. ↑ Ursel Laarmann, Michael Landmesser, Maximilian Glas, Rupert Hochleitner, Rudolf Dröschel, Peter Jeckel: Agate: The precious stone from which Idar-Oberstein was created: history, development, finds . In: Christian Weise (ed.): ExtraLapis . tape 19 . Christian Weise Verlag, 2000, ISBN 3-921656-54-0 , ISSN  0945-8492 .
  4. ^ Johann Zenz: Agates . Bode, Salzhemmendorf-Lauenstein 2005, ISBN 3-925094-82-2 .
  5. ↑ Michael Landmesser: The riddle agate . In: Official catalog of the Mineralientage Munich . 1987, p. 65-88 .
  6. ↑ Michael Landmesser: Construction and formation of agates . In: Lapis . 13th year, no. 9 , 1988, pp. 11-28 .
  7. ↑ Michael Landmesser: How are agates made? In: Agate: The precious stone from which Idar-Oberstein was created: history, origin, finds (=  extraLapis . No. 19 ). 2000, p. 58-73 .
  8. ↑ Peter Riethe : Hildegard von Bingen. The book of the stones . Translated from the sources and explained by Peter Riethe. 4th edition. Otto Müller Verlag, Salzburg 1997, ISBN 3-7013-0946-9 , p.  101-104 .
  9. ^ Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 290 .
  10. ^ Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 284-286 .
  11. ↑ Jaroslav Bauer, Vladimír Bouška, František Tvrz: Gemstone Guide . Verlag Werner Dausien, Hanau / Main 1993, ISBN 3-7684-2206-2 , p. 211 .