sedimentary
Chert (Quartz Variety)
Microcrystalline / Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brownish-gray with white mottling; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Brownish-gray with white mottling
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brownish-gray with white mottling; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary beds. This waterworn specimen likely weathered out of limestone or chalk.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Today used as decorative gravel, in construction aggregate, and for river stone landscaping.
Geological facts
Chert is so durable that it often survives long after the surrounding rock has eroded away. Flint is a high-quality, dark variety of chert used historically for weaponry.
Field identification & locations
Identify by its 'waxy' texture, extreme hardness (it will scratch glass), and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds and glacial tills.
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