sedimentary
Chert (Gravel)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs); Color: Gray, tan, white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs)
- Color
- Gray, tan, white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs); Color: Gray, tan, white; 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 diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. This specimen is a weathered piece of gravel found in concrete or aggregate.
Uses & applications
Used primarily in construction as aggregate for concrete and roadbeds. Historically used for making stone tools due to its sharp conchoidal fracture.
Geological facts
Chert is so hard it can strike sparks against steel. It is one of the most common materials used by prehistoric humans for arrowheads and scrapers.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy appearance, and smooth, curved (conchoidal) breaks. Often found in riverbeds or as component gravel in landscaping and driveways.
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