Fairburn Agate

The Fairburn Agate is a type of gemstone found in the agate beds of South-Western South Dakota and North-Western Nebraska. It is also the state gemstone of South Dakota.[1] Fairburns are characterized from other types of Agate by their colors and the shape of the bands.[1][2]

Fairburn Agate
Fairburn Agate showing the fortification matrix
General
CategoryMineral
Formula
(repeating unit)		SiO2 silicon dioxide
Identification
Colorbrown, red, orange, blue, yellow, black, and white banded
Crystal habitCryptocrystalline silica
Mohs scale hardness6.5–7
LusterWaxy

Distinction

Fairburns, like other types of Agate, 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.[1][2]

Buffalo Gap National Grasslands, the location of the fairburn agate beds

Formation

The genesis of fairburn agates is debated and not fully understood, however two theories are most accepted.[3][2]

Theory 1

Fairburn formation takes place in sedimentary limestone, where calcite inside the limestone is replaced by silicon-dioxide or silicic acid over time in a high pressure, micropourous environment.[1] In this theory, bands of agate formed from the inside of the agate outwards.[2]

Theory 2

This theory goes against physical evidence and is not accepted by most researchers.

The agates are created by heat and pressure filling cavities in igneous rock with layers of silica over time.[3] The first layer of silica would line the inside of the cavity and crystallize. Then, during another geologic event, a different set of minerals and silica would 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.[3] This theory goes against physical evidence and is not accepted by most.

Widely Accepted Facts

Colors found in the bands are created by trace amounts of minerals such as iron oxide, hematite, goethite, magnetite, and other oxidated minerals.[1] These materials mixed with the silica and were included in the crystallization during formation, giving the layers of cryptocrystalline 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, and were exposed due to erosion and 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.[1][2][3]

Cultural Significance

Fairburns as well as other types of agates are collected by geologists and other interested individuals for pleasure or use in jewelry or lapidary artwork.[3] Yearly conventions are held in places such as Crawford, Nebraska, for collectors to exchange rocks and socialize with other collectors. Often during erosion, agates are broken in half. These festivals and conventions give collectors a chance to find the missing half of an agate they found.[3]

The Fairburn has also been designated as the state gemstone of South Dakota as of February 11, 1966, due to its prominence in the region and cultural impact with regards to collecting and decoration.[4]

References
  1. Clark, Roger. (2009). South Dakota's State Gemstone : [autographed copy] ; Fairburn Agate. Clark, Mary, Jane. (1st ed.). Appleton, WI: Siverwind Agates. ISBN 9780966464023. OCLC 698895703.
  2. Clark, Roger (Roger W.) (1998). South Dakota's Fairburn agate (1st ed.). Appleton, WI: Silverwind Agates. ISBN 0966464001. OCLC 39727762.
  3. Clark, Roger (Roger W.) (2002). Fairburn agate : gem of South Dakota (1st ed.). Appleton, WI: Silverwind Agates. ISBN 096646401X. OCLC 51759315.
  4. "Fairburn Agate". www3.northern.edu. Retrieved 2019-06-14.
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