sedimentary
Massive Cryptocrystalline Quartz (Chert/Chalcedony Pebble)
Silicon Dioxide (SiO2)
Hardness: 6.5 - 7 (Mohs scale); Color: Creamy white with grey/tan mottling; Luster: Waxy to dull; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.65.
- Hardness
- 6
- Color
- Creamy white with grey/tan mottling
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5 - 7 (Mohs scale); Color: Creamy white with grey/tan mottling; Luster: Waxy to dull; Structure: Cryptocrystalline (microscopic crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.65.
Formation & geological history
Formed through the accumulation of silica-rich fluids in limestone/chalk cavities or through the accumulation of silica-rich organic remains (like radiolaria). This specific specimen is a 'water-worn' pebble, likely tumbled in a river or coastal environment during the Quaternary period.
Uses & applications
Historically used for stone tools (flintknapping); currently used as a common architectural aggregate, road ballast, or for craft jewelry (tumbling).
Geological facts
Chert is one of the most durable rocks on the planet, often outlasting the bedrock it formed in. It has been used by humans for over 2 million years to create sharp-edged tools and to start fires due to its ability to produce sparks when struck against iron pyrites or steel.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy feel, and rounded 'river-cobble' shape. Commonly found in gravel pits, riverbeds, and coastal beaches worldwide, particularly in areas with limestone bedrock.
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