sedimentary
Chert (often called Jasper or River Rock)
Chert (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Tan, brown, and grey with some banding; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8
- Hardness
- 6
- Color
- Tan, brown, and grey with some banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Tan, brown, and grey with some banding; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline (Trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation from groundwater within sedimentary layers. These specimens are typically tumbled and rounded by water action in rivers or glacial deposits.
Uses & applications
Used historically for stone tools due to conchoidal fracture. Modernly used in landscaping, as aquarium stones, or for lapidary work and rock tumbling.
Geological facts
This specimen shows natural fractures and water-worn surfaces. Chert is extremely durable, which is why it often survives long-distance transport in rivers and glacial till while softer rocks erode away.
Field identification & locations
Identify by its hardness (will scratch glass), waxy luster when wet or sun-bleached, and smooth, rounded shape if found in water. Common across North America in glacial drift and riverbeds.
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