What is AgateAgate is a type of chalcedony, which is a microcrystalline form of quartz. It is known for its beautiful banding and patterns, which are caused by the different impurities that are present in the stone. Agate can be found in a wide range of colors, including blue, green, red, yellow, and white.
Agate is formed when silica-rich water fills rock voids, particularly volcanic rock voids. As the water leaves, silica mineral deposits remain, and they functionally form gemstone crystals. Over time, numerous layers develop, leading to an agate.
Agate NameThe 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 to Find Agate
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.
Types of AgateThere 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.
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
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 agate from Pariaba, Brazil.|
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.
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).
The Lake Superior Agate
|Agate Lake Superior|
Photo Copyright © Lech Darski
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 Agate is an agate with acicular or or pointed inclusions of various minerals (mostly rutile). These hair like formations are often arranged in fans or bursts. The crystals are arranged in a fan-like or sunburst pattern, which gives the stone its distinctive appearance. Sagenite agate is found in many parts of the world, including Brazil, Mexico, and the United States. It is a popular gemstone for jewelry and other decorative items.
Fortification agate is a type of agate that is characterized by its distinctive banding pattern. The bands are typically sharp-angled and often resemble the outlines of fortifications, such as a castle or a fort. Fortification agates can be found in a variety of colors, including white, yellow, orange, red, brown, and black. They are typically found in sedimentary rock formations, such as sandstone and limestone.
Fortification agates are prized for their beauty and rarity. They are often used in jewelry and other decorative items. Some fortification agates are also considered to be good luck charms.
|Fairburn agates Locality: Rapid City, South Dakota, United States |
Photo: © Captain Tenneal
Fairburn Agate is 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.
Fairburn agates are composed of concentric layers of cryptocrystalline chalcedony colorized by different trace minerals. However, fortification banding distinguishes fairburns from other agate types. Fortification banding means that the concentric layers have sharp changes in direction which cause the bands to form angles in ways which are especially distinguishable from other agate types. Mainly, no other agate type forms these patterns.
Fairburn agates are typically found in a variety of colors, including red, orange, yellow, green, blue, and purple. They can also be found in a variety of patterns, including fortification banding, dendritic banding, and mottled banding.
|A Slab of Agate From Botswana. |
Photo by Avegaon on Flickr
|Dendritic Agate from India|
Dendritic agate is a popular gemstone and is used in jewelry. It is characterized by its tree-like or fern-like patterns, which are caused by the presence of manganese or iron oxide minerals. Dendritic agate is formed when silica-rich groundwater flows through cracks and fissures in rocks. The silica slowly precipitates out of the water and forms a dendritic pattern. The exact mechanism by which this occurs is not fully understood, but it is thought to be related to the oxidation of iron or manganese minerals.
|Coyamito Agate Pseudomorphs|
Coyamito Agate Pseudomorphs a pseudomorph is something that takes the form of something else. Pseudomorphs in agate are quite rare but do occur in nodular agates from various locations, usually as a calcite or aragonite replacement. The Coyamito agate deposit in Chihuahua Mexico produces more pseudomorphs than any other agate location.
|Natural Plume agate|
Plume agate is a type of agate that is characterized by its featherlike inclusions. These inclusions can be a variety of colors, including white, gray, black, and brown. Plume agate is a relatively rare type of agate, and it is often more expensive than other types of agate. Plume agate is formed when iron oxides are deposited in agate in a way that resembles feathers or smoke. The iron oxides can be deposited in a variety of ways, including through volcanic activity, hydrothermal activity, and weathering. It is a rarer type of agate and is often more expensive than other types of agate.