sedimentary
Chert Pebble Map
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Tan, brown, and gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Tan, brown, and gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Tan, brown, and gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed as nodules in limestone or chalk through the accumulation of silica-rich organic remains (like radiolaria or diatoms) that recrystallize over millions of years.
Uses & applications
Historically used for stone tools and fire-starting; currently used as construction aggregate, road surfacing, or as decorative river rock.
Geological facts
Chert is extremely durable and chemically stable, which is why it is often found as rounded pebbles in stream beds long after the surrounding limestone has eroded.
Field identification & locations
Identify in the field by its waxy texture and sharp, shell-like (conchoidal) edges when broken. It is harder than steel and will scratch a glass plate. Commonly found in riverbeds and gravel pits.
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