sedimentary
Chert (Flint)
Microcrystalline or cryptocrystalline quartz (SiO2)
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
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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.
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