Chert (Brian Head Formation, Middle Eocene to Lower Oligocene; Brian Head Mountain, Iron County, Utah, USA) 3
Chert with a whitish-colored rind of tripoli/rottenstone from the Tertiary of Utah, USA.
Chert (a.k.a. "flint") is a cryptocrystalline, quartzose sedimentary rock. Rockhounds and some geologists assert that flint is high-quality while chert is low-quality. Some geologists assert that "flint" implies a biogenic origin and "chert" implies a chemical origin.
Many cherts do have a chemical origin - chert nodules are moderately common in some limestone units. The nodules form during diagenesis - pre-existing siliceous components in the carbonate sediments are dissolved, mobilized, and reprecipitated as chert masses. Some cherts do have a biogenic origin - for example, radiolarian cherts (rich in radiolarian microfossils) or spicular cherts (rich in siliceous sponge spicules).
Stratigraphy: Brian Head Formation (apparently from a micritic limestone interval), upper Middle Eocene to Lower Oligocene
Locality: loose piece adjacent to chert meganodule in hillside on the eastern side of Rt. 148, southwestern side of Brian Head Mountain, between the town of Brian Head and Cedar Breaks National Monument, southeastern Iron County, southwestern Utah, USA