Agate 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.
Cryptocrystalline means that the mineral crystals are so fine that they are only vaguely seen in patterns and fine variations.
Where to Find Agate
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.
Agate Types and Varieties
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 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
Crazy Lace Agate is a variety of banded Chalcedony, It is predominantly white, with layers of creamy browns, blacks and grays. Some may include layers of yellow ochre, gold, scarlet and red. Agate is sometimes called the earth rainbow because, in its various forms, the concentric bands in nature form nearly every color the earth can produce, including a colorless form.
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 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).
Despite its name, moss agate does not contain organic matter and is usually formed from weathered volcanic rocks.
The Lake Superior Agate
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 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
Botswana Agate is a type of banded agate that is found exclusively in Botswana, a country located in southern Africa. It is known for its intricate banding patterns, which are a result of the stone forming in layers of volcanic ash. Botswana Agate is typically found in shades of white, black, gray, and blue, but it can also be found in other colors, such as red and pink.
|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.
Agate is a popular gemstone that is used in a variety of jewelry and other decorative objects. It is also a popular rock for collectors.