sedimentary

Chert Arrowheads

Microcrystalline Quartz (SiO2)

Hardness of 7 on the Mohs scale, waxy luster, conchoidal fracture, and colors ranging from tan to dark grey-black.

Identified November 11, 2026More sedimentary →

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Physical properties

Hardness of 7 on the Mohs scale, waxy luster, conchoidal fracture, and colors ranging from tan to dark grey-black.

Formed via the chemical replacement of limestone or chalk by silica-rich groundwater over millions of years, later knapped by humans into artifacts.

Used historically by indigenous peoples for hunting and toolmaking, and collected today as valuable archaeological artifacts.

Chert and flint were prized globally for their predictable conchoidal fracture, allowing precise control when shaping sharp edges.

Identified by smooth, concave flake scars and worked notches, commonly found near ancient waterways and archaeological sites in Texas.

Fossil-bearing concretion, possibly containing a fossil shell or other organic inclusion

Calcareous concretion; exact fossil and host-rock composition are not identifiable from the image

Fossiliferous Sedimentary Rock

Sedimentary limestone with invertebrate fossil imprint

Sandstone or Siltstone

Arenite (if sandstone)

Sandstone

Arenite

Shale

Shale (clastic sedimentary rock)

Shale (or Mudstone/Siltstone)

Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)