sedimentary
Chert (River Pebble/Finger Stone)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grayish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation within marine or lacustrine sedimentary rocks. This specimen is a water-worn pebble likely transported by fluvial processes.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used in construction aggregate and as a collector's pocket stone.
Geological facts
Chert is so durable that it often survives long after the surrounding limestone or chalk has eroded away. Specially shaped stones like this are sometimes referred to as 'finger stones' due to how they fit in the hand.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, waxy texture. Commonly found in riverbeds, gravel pits, and coastal areas.
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