sedimentary

Fairburn Agate

Silicon Dioxide (SiO2) - Chalcedony / Cryptocrystalline Quartz

Hardness: 6.5-7 (Mohs scale), Color: Red, orange, white, and yellow bands with sharp patterns, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.60–2.65

Hardness: 6

Identified July 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Oligocene epoch (approx. 24-34 million years ago). Siliceous fluids filled cavities in sedimentary limestone or sandstone through groundwater precipitation, later weathering out of their host rock into alluvial deposits.

Uses & applications

Primarily used for high-end lapidary work, jewelry, and as a highly prized collector's specimen. It is the state gemstone of South Dakota.

Geological facts

Fairburn agates are famous for their unique 'fortification' patterns, featuring sharp-angled concentric banding that looks like the aerial view of a fort. They are among the most sought-after agates in the world due to their intricate detail and rarity.

Field identification & locations

Found in the Fairburn Agate Beds and surrounding areas of the Black Hills in South Dakota and Nebraska. Best identified by the sharp, zig-zagging fortification bands and waxy texture. Collectors should look in gravel bars and dry creek beds.

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Sandstone or Siltstone

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Fossiliferous Concretion

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Brown Sandstone (River Rock)

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Mudstone

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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