sedimentary
Chert (River Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Brown, tan, reddish-brown with banded patterns; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Brown, tan, reddish-brown with banded patterns
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Brown, tan, reddish-brown with banded patterns; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed primarily in marine environments from the accumulation of siliceous organisms or silica-rich chemical precipitates. Over millions of years, heat and pressure transform the silica into microcrystalline quartz. This specimen shows smoothing from fluvial erosion (river transport).
Uses & applications
Historically used for making stone tools (arrowheads); currently used in construction aggregate, road base, and as decorative river stones in landscaping. Highly polished specimens are used in tumblers for lapidary hobbies.
Geological facts
Chert has been used by humans for hundreds of thousands of years because it breaks with a razor-sharp edge. When struck against steel, it can create sparks, similar to flint.
Field identification & locations
Identify in the field by its smooth, water-worn texture, variety of earthy colors, and extreme hardness (it will easily scratch glass). Common in riverbeds and gravel pits worldwide.
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