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Agate is a cryptocrystalline variety of silica, chiefly chalcedony, characterised by its fineness of grain and brightness of color.

Cryptocrystalline means that the mineral crystals are so fine that they are only vaguely seen in patterns and fine variations.

Name:The name is derived from its occurrence at the Achates River in southwestern Sicily.A distinctly banded fibrous chalcedony. Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.

Where found:
Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls. Agate has also been known to fill veins or cracks in volcanic or altered rock underlain by granitic intrusive masses. Such agates, when cut transversely, exhibit a succession of parallel lines, often of extreme tenuity, giving a banded appearance to the section. Such stones are known as banded agate, riband agate and striped agate.

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 drusy quartz, sometimes amethystine, having the apices of the crystals directed towards the free space so as to form a crystal-lined cavity or geode.

Note: Agate is not simply "banded chalcedony." There are other types of chalcedony that are banded that do not match the description above, banded flint, for example.
There are a number of varieties of chalcedony that are called "agate" that do not match the definition given above. Good examples are "feather agates" and "fire agates". These are listed as varieties of chalcedony, not as varieties of agate.

Two characteristic types of banding can be distinguished in agates:
1. Wall-lining Banding. The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibers. Since the chalcedony fibers grow from the walls to the interior of a cavity, a concentric, onion-like pattern develops.

2. Horizontal Banding (also called Uruguay-type banding). This type of banding is less common, and usually accompanied by wall-lining banding. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitated in the cavity. Horizontal bands can serve as spirit levels to determine the original orientation of the specimen in the host rock. When the difference in translucency or color between the layers is pronounced, agates with horizontal banding can be used for cutting cameos and engravings.





There will probably be no comprehensive list of all possible names for Agate, because old names number in the hundreds and new names are coined frequently, mostly with no involvement by scientists who go through the rigors of properly classifying the rocks. Some variety names are generally used by collectors and dealers, but there are many made up by dealers to describe a locality or other habit. The varieties below are the well-known names or varieties that are commonly encountered. Seldom-used and localized trade names are not described here.
Onyx-Agate from Minas Geraís, Brasil. Collection:Technical University Kosice.
Photography by Ivan Karas
Onyx is formed of bands of chalcedony in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.  The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx contain bands of black and/or white.
Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black.
Iris Agate in the sun,Wood's Stoneworks and Photo Factory on flickr
Iris Agate is a name used for a finely-banded agate that produces a spectacular display of color when it is cut properly and illuminated from a direction that sends light through its very thin bands. The name "iris agate" is used because one meaning of the word "iris" is "a rainbow-like display of colors."
Crazy lace agate From MEXICO, Chihuahua, Mun.
de Buenaventura, Sierra Santa Lucia, Ejido Benito Juarez, Credit: Anton
Crazy Lace Agate An agate composed of multicoloured twisting and turning bands.

Thunder egg is a nodule-like rock, similar to a filled geode,
that is formed within volcanic ash layers. Credit: Bill The Eggman

 Thunder-egg is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. Thundereggs are rough spheres, most about the size of a baseball- though they can range from less than an inch to over a meter across.
 

They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions.




Translucency of the Brazilian enhydro agate.
Credit:
The Agatelady
Enhydro agate are nodules, agates, or geodes with water trapped inside its cavity. Enhydros are closely related to fluid inclusions, but are composed of chalcedony. The formation of enhydros is still an ongoing process, with specimens dated back to the Eocene Epoch. They are commonly found in areas with volcanic rock.
Rare to find multi-chambered specimen of Polyhedroid agatePariaba, Brazil.
Photo: Dwarves' Earth Treasures Museum
Polyhedroid agate is agate which has grown in a flat-sided shape similar to a polyhedron. When sliced, it often shows a characteristic layering of concentric polygons. Polyhedroid agate is thought to be found only in Paraíba State, Brazil. It has been suggested that growth is not crystallographically controlled but is due to the filling-in of spaces between pre-existing crystals which have since dissolved.
A piece of colorful moss agate stone from indonesia..
Credit Yudi Aswanto
Moss agate is a semi-precious gemstone formed from silicon dioxide. It is a form of chalcedony which includes minerals of a green colour embedded in the stone, forming filaments and other patterns suggestive of moss. The field is a clear or milky-white quartz, and the included minerals are mainly oxides of manganese or iron. It is not a true form of agate, as it lacks agate's defining feature of concentric banding.

Moss agate is of the white variety with green inclusions that resemble moss. It occurs in many locations. The colors are formed due to trace amounts of metal present as an impurity, such as chrome or iron. The metals can make different colors depending on their valence (oxidation state).
Despite its name, moss agate does not contain organic matter and is usually formed from weathered volcanic rocks.
Agate Lake Superior,Copyright © Lech Darski
The Lake Superior agate is a type of agate stained by iron and found on the shores of Lake Superior. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota, Wisconsin, and Michigan. The Lake Superior agate was selected because the agate reflects many aspects of Minnesota. It was formed during lava eruptions that occurred in Minnesota about a billion years ago.

The stone's predominant red color comes from iron, a major Minnesota industrial mineral found extensively throughout the Iron Range region. Finally, the Lake Superior agate can be found in many regions of Minnesota as it was distributed by glacial movement across Minnesota 10,000 to 15,000 years ago.




Condor agate from Argentina
Condor agate was discovered and named by Luis de los Santos in 1993. It is found in the mountains near San Rafael, in Mendoza Province, Argentina. This agate exhibits vibrantly colorful bands and patterns, and has become a popular stone among collectors and jewelry designers.
 Sagenite Laker agate. Credit: Captain Tenneal
Sagenite Agate Agate with acicular or or pointed inclusions of various minerals. These hair like formations are often arranged in fans or bursts.

Fortification Agate, Amethyst Cove. Credit: Christopher Sheppard
Fortification Agate Agate with sharp-angled bands which resemble the outlines of fortifications of a castle.
Fairburn agates  Locality: Rapid City, South Dakota,
United States Photo: ©Captain Tenneal
Fairburn Agate: A unique and rare variety of Fortification Agate from Fairburn, Custer Co., South Dakota, USA. The state gem of South Dakota. Formed in Pennsylvanian-Permian carbonate sediments and weathered out since Oligocene around the Black Hills. Agates in Nebraska formed in Pennsylvanian sediments and were transported from the Hartville uplift in Wyoming in the Oligocene.

References:
Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. Reviews in Mineralogy, Vol.29, Silica - Physical behavior, geochemistry and materials applications.
 Plodowski, G., and Werner, R.; Achate: Kleine Senckenberg-Reihe, No. 12
Agate wikipedia 
 Mindat

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