sedimentary

Flint

Cryptocrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale, typically brown to dark grey or black in color with a waxy to vitreous luster and conchoidal fracture.

Identified November 12, 2026More sedimentary →

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Physical properties

Hardness of 7 on the Mohs scale, typically brown to dark grey or black in color with a waxy to vitreous luster and conchoidal fracture.

Formed through the chemical replacement of calcium carbonate in limestone or chalk by silica-rich groundwaters over millions of years.

Historically used for making stone tools and striking fire; currently used in decorative landscaping, abrasive grits, and ball mills.

Flint is extremely durable and breaks with sharp, curved edges, making it ideal for ancient weaponry and tools.

Found widely as nodules within chalk and limestone formations globally, easily identified by its conchoidal fracture and waxy rind.

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

Sandstone or Siltstone

Arenite (if sandstone)

Fossiliferous Sedimentary Rock

Sedimentary limestone with invertebrate fossil imprint

Sandstone

Arenite

Shale

Shale (clastic sedimentary rock)

Shale (or Mudstone/Siltstone)

Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)