sedimentary
Chert (often called Flint)
Chert (Cryptocrystalline Silica, SiO2)
Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Waxy to dull
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed by the precipitation of silica from groundwater or within seafloor sediments, often replacing organic matter or calcium carbonate. Many specimens date to the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools (knapping) and fire-starting; currently used in construction aggregate, road surfacing, and as a semi-precious gemstone in jewelry.
Geological facts
Flint is a variety of chert that occurs in chalk or marly limestone. It was the primary material for spearheads and arrowheads in prehistoric times due to its predictable conchoidal fracturing.
Field identification & locations
Identify by its waxy appearance, extreme hardness (scratches glass), and shell-like (conchoidal) curved fracture patterns. Commonly found in limestone riverbeds or gravel deposits.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary