sedimentary
Chert (Flint)
Chert (Cryptocrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical precipitation of silica in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used in road construction, as an abrasive, and sometimes in landscaping or decorative masonry.
Geological facts
Because it breaks with a conchoidal fracture, creating sharp edges, chert was the primary material for arrowheads and spear points for thousands of years. It is essentially a microcrystalline version of quartz.
Field identification & locations
Identify in the field by its waxy appearance and the shell-like (conchoidal) curves where it has broken. It is very hard and will easily scratch glass. Found worldwide in sedimentary basins.
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