sedimentary
Chert (variously identified as Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Black, grey, white, and tan; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Black, grey, white, and tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Black, grey, white, and tan; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Modern uses include road construction aggregate, fire-starting strikers, and as a decorative stone for lapidary work.
Geological facts
Chert was one of the first materials used by early humans to create sharp tools due to its conchoidal fracture. It is extremely chemically stable and resistant to weathering, which is why it often appears as rounded river or beach pebbles.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or coastal areas where softer surrounding rock has eroded away.
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