The Fairburn Agate is a type of

gemstone A gemstone (also called a fine gem, jewel, precious stone, semiprecious stone, or simply gem) is a piece of mineral crystal which, when cut or polished, is used to make jewellery, jewelry or other adornments. Certain Rock (geology), rocks (such ...

found in the agate beds of Southwestern

South Dakota South Dakota (; Sioux language, Sioux: , ) is a U.S. state, state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota people, Dakota Sioux ...

and Northwestern

Nebraska Nebraska ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Ka ...

. It is also the state gemstone of South Dakota. Fairburns are characterized from other types of

agate Agate ( ) is a banded variety of chalcedony. Agate stones are characterized by alternating bands of different colored chalcedony and sometimes include macroscopic quartz. They are common in nature and can be found globally in a large number of d ...

by their colors and the shape of the bands.

Distinction

Fairburns, like other types of agate, are composed of

concentric In geometry, two or more objects are said to be ''concentric'' when they share the same center. Any pair of (possibly unalike) objects with well-defined centers can be concentric, including circles, spheres, regular polygons, regular polyh ...

layers of

cryptocrystalline Cryptocrystalline is a rock microstructure, rock texture made up of such minute crystals that its crystalline nature is only vaguely revealed even microscopically in thin section by transmitted polarized light. Among the sedimentary rocks, chert a ...

chalcedony Chalcedony ( or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic ...

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. Buffalo Gap National Grassland

Formation

The genesis of fairburn agates is debated and not fully understood; however, two theories are most accepted.

Theory 1

Fairburn formation takes place in

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

, where calcite inside the limestone is replaced by silicon-dioxide or

silicic acid In chemistry, a silicic acid () is any chemical compound containing the element silicon attached to oxide () and hydroxyl () groups, with the general formula or, equivalently, . Orthosilicic acid is a representative example. Silicic acids are ra ...

over time in a high-pressure, micropourous environment. In this theory, bands of agate form from the inside of the agate outwards.

Theory 2

The agates are created by heat and pressure filling cavities in

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

rock with layers of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

, over time. The first layer of silica lines the inside of the cavity and crystallizes. Then, during another geologic event, a different set of minerals and silica form on top of these layers, essentially forming the layers from the outside in, explaining why some agates have cavities in the center. These agates are incomplete and have not been fully filled with silica.

Widely accepted facts

Colors found in the bands are created by trace amounts of minerals such as

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

, and other oxidated minerals. These materials mixed with the silica and were included in the

crystallization Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...

during formation, giving the layers of

quartz their color.

Formation location

It is suspected that fairburn agates originated in or near the Black Hills of Eastern Wyoming and Western South Dakota, were exposed due to erosion, and were carried into agate beds downstream. It is also possible they formed at the site where they lie and were washed free of dirt and limestone debris by wind and weather. However, the former is more plausible than the latter.

Cultural significance

Fairburns, as well as other types of agates, are collected by geologists and other interested individuals for pleasure or use in

jewelry Jewellery (or jewelry in American English) consists of decorative items worn for personal adornment such as brooches, ring (jewellery), rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the ...

lapidary Lapidary () is the practice of shaping rock (geology), stone, minerals, or gemstones into decorative items such as cabochons, engraved gems (including cameo (carving), cameos), and faceted designs. A person who practices lapidary techniques of ...

artwork. Yearly conventions are held in places such as

Crawford, Nebraska Crawford is a city in Dawes County, Nebraska, Dawes County, Nebraska, United States, in the Great Plains region. The population was 997 at the 2010 United States Census, 2010 census. It was incorporated in 1886 and was named for the late Captain ...

, for collectors to exchange rocks and socialize with other collectors. Often during erosion, agates were broken in half. These festivals and conventions give collectors a chance to find the missing half of an agate they found. The fairburn was designated as the state gemstone of South Dakota as of February 11, 1966, due to its prominence in the region and cultural impact with regard to collecting and decoration.

References

{{Reflist Agates Geology of South Dakota