sedimentary
Chert (Jasper/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, and reddish-brown banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, and reddish-brown banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, and reddish-brown banding; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich fluids in sedimentary environments or by the replacement of carbonate minerals. This specific water-worn pebble likely originated from an ancient riverbed or coastal deposit, smoothed over thousands of years.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to conchoidal fracturing. Modern uses include landscaping, tumbling for hobbyists, and occasional use in inexpensive jewelry as 'river stones'.
Geological facts
Chert is so durable that it often survives long after the surrounding rock has eroded away. It is chemically identical to quartz but has a much finer grain structure, making it tougher and less prone to shattering.
Field identification & locations
Identify in the field by its waxy texture, smooth water-worn surface, and inability to be scratched by a steel knife. Commonly found in gravel pits, riverbeds, and along beaches. It is a favorite for beginning rock tumblers.
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