sedimentary

Chert (Flint)

Microcrystalline or cryptocrystalline quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: tan, brown, gray, or yellow; Luster: waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: none, exhibits conchoidal fracture; SG: 2.65.

Hardness: 7 on Mohs scale
Color: tan, brown, gray, or yellow
Luster: waxy to dull

Identified April 6, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: tan, brown, gray, or yellow; Luster: waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: none, exhibits conchoidal fracture; SG: 2.65.

Formation & geological history

Formed in marine environments through the accumulation of silica-rich biogenic ooze (diatoms/radiolarians) or the replacement of limestone by silica-rich fluids. Can be found in nodules within chalk or limestone layers.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used as road surfacing, decorative gravel, and occasionally for lapidary work.

Geological facts

Flint is a variety of chert favored by ancient humans for making arrowheads and spear points because it breaks with incredibly sharp edges. It can also produce sparks when struck against steel.

Field identification & locations

Identify by its waxy luster, extreme hardness (scratches glass), and distinctive shell-like (conchoidal) curved fracture patterns. Commonly found in river beds or eroded from limestone cliffs.