sedimentary
Chert (Gravel)
Chert (Cryptocrystalline silicon dioxide, 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 siliceous remains of organisms (like diatoms or radiolarians) or by chemical precipitation. Often occurs as nodules within limestone or dolostone layers from various geological ages, often Paleozoic or Mesozoic.
Uses & applications
Used historically for stone tools due to its sharp edges; currently used in construction as road aggregate, gravel, and occasionally for lapidary work if highly figured (like jasper).
Geological facts
Chert is so hard it can scratch steel and glass. It was used by indigenous cultures worldwide to make arrowheads and spear points because of its predictable conchoidal fracturing pattern.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a knife), smooth waxy feel, and characteristic shell-like (conchoidal) fractures. Common in riverbeds and glacial till.
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