sedimentary
Flint with Chalk Cortex
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale), Color: Black to dark gray interior with a white/cream exterior, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Fracture: Conchoidal
Identified More sedimentary →
Explore Flint with Chalk Cortex in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 7 (Mohs scale), Color: Black to dark gray interior with a white/cream exterior, Luster: Waxy to vitreous, Structure: Cryptocrystalline, Fracture: Conchoidal
Formation & geological history
Formed in marine sedimentary environments, specifically within Cretaceous chalk beds, by the replacement of calcium carbonate with silica from sponges or diatoms.
Uses & applications
Historically used for stone tools and starting fires; currently used for lapidary work, landscaping, and primitive technology demonstrations.
Geological facts
Flint nodules are often found with a white, porous outer layer called a cortex, which is a mixture of silica and calcium carbonate remnants from the surrounding chalk.
Field identification & locations
Look for a smooth, rounded or irregular nodule with a white 'skin'. If broken, it reveals a dark, glass-like interior. Commonly found on beaches near chalk cliffs like those in Dover, UK.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sedimentary Sandstone
Arenite (Sedimentary Rock)
sedimentary
Fossiliferous Concretion
Septarian Concretion containing Brachiopods/Crinoids
fossil
Shale
Shale (clastic sedimentary rock)
sedimentary
Brown Sandstone (River Rock)
Arenite
sedimentary
Shale
Argillaceous sedimentary rock
sedimentary