sedimentary

Flint in Chalk

Silicon dioxide nodules in microcrystalline calcium carbonate

Mohs hardness of 7 for the flint core and 3 for the chalk exterior. Exhibits conchoidal fracture with a waxy to vitreous luster.

Identified October 20, 2026More sedimentary →

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Physical properties

Mohs hardness of 7 for the flint core and 3 for the chalk exterior. Exhibits conchoidal fracture with a waxy to vitreous luster.

Formed through the diagenesis of silica-rich marine organism remains accumulating in chalk beds during the Cretaceous period.

Historically used for making stone tools and fire-starting; today collected as a geological specimen.

Flint is extremely durable and breaks with razor-sharp edges, making it ideal for prehistoric weaponry and cutting tools.

Found embedded within white chalk cliffs or deposits worldwide; identified by its hard, dark core contrasting with soft, chalky matrix.

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)