Fairburn agate is the trade name for agates that come from the agate beds of Southwestern South Dakota and Northwestern Nebraska. The agate often bears fortification banding, meaning there are sharp angled bands as well as small projections that come off of these angles, resembling an overhead view of a castle wall and its fortifications. There are two accepted theories as to how these agates formed, one of which indicates that they formed in sedimentary limestone, while the other theorizes that they formed within cavities of igneous rock. It's widely believed that they eroded out of the Black Hills of Eastern Wyoming and Western South Dakota and were deposited by river into these agate beds, however it's possible that they actually formed in said agate beds, only for the rock to be weathered away. Their red to brown-orange coloration is primarily a result of goethite, magnetite and hematite impurities within the silica-rich fluids, among other oxide minerals.
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