sedimentary
Chert (Flint Pebbles)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, white, and brown with a waxy to dull luster; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, and brown with a waxy to dull luster
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, and brown with a waxy to dull luster; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds. This specific specimen shows signs of fluvial or coastal tumbling, creating its rounded pebble shape.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to its sharp edges when fractured. Modern uses include road aggregate, high-end landscaping, and as a raw material for silicon production.
Geological facts
Chert is so hard it can scratch glass and steel. It was one of the first materials used by early humans for toolmaking and starting fires (flint and steel). The white patches on the surface are typically a weathered 'patina' or cortex where the silica has leached or reacted with mineralized groundwater.
Field identification & locations
Identify in the field by its smooth, waxy feel and distinctive 'U' shaped (conchoidal) chips or fractures. It is commonly found in riverbeds, beaches, and sedimentary basins worldwide, particularly where limestone is present.
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