sedimentary
Chert Pebble (Beach Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale. Color: Greyish-tan to light brownish grey. Luster: Dull to smooth/waxy when weathered or beach-rolled. Crystal Structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific Gravity: 2.58-2.64.
- Hardness
- 7 on Mohs scale
- Color
- Greyish-tan to light brownish grey
- Luster
- Dull to smooth/waxy when weathered or beach-rolled
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Physical properties
Hardness: 7 on Mohs scale. Color: Greyish-tan to light brownish grey. Luster: Dull to smooth/waxy when weathered or beach-rolled. Crystal Structure: Trigonal (microcrystalline/cryptocrystalline). Cleavage: None, exhibits conchoidal fracture. Specific Gravity: 2.58-2.64.
Formation & geological history
Formed from biochemical precipitation of silica or accumulation of siliceous organism remains (like radiolarians or diatoms) on the ocean floor, followed by diagenesis into dense nodular chert or flint. Later weathered, eroded, and rounded by wave or river action into a smooth pebble.
Uses & applications
Historically used for making stone tools (flint knapping) and sparking fire. Today used primarily in aggregate materials, landscaping, or as decorative pebbles.
Geological facts
Chert pebbles are among the most durable rocks found along beaches and riverbeds because silica resists chemical weathering and physical abrasion very well. Small pits on the surface are often evidence of tiny dissolved mineral inclusions or bio-erosion.
Field identification & locations
Look for a smooth, rounded surface with small pinhole cavities and high hardness (will scratch glass or steel). Commonly found on gravel beaches, riverbeds, and glacial till deposits worldwide.
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