sedimentary
Chert Pebble with Quartz Veins
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to brown with white veins, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey to brown with white veins, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.65
Formation & geological history
Formed as chemical precipitates or through the replacement of limestone. This specimen shows secondary brittle fracturing followed by quartz mineralization (veining) within the host rock, subsequently water-worn into a pebble shape.
Uses & applications
Predominantly used as architectural aggregate or road gravel. Historically used for making stone tools (arrowheads). Small water-worn pebbles are popular for rock tumbling and aquarium decor.
Geological facts
Chert is so hard it can produce sparks when struck against steel. The white 'X' or criss-cross pattern is formed when tectonic stress cracks the rock and silica-rich fluids seep in and crystallize as quartz.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and waxy texture. Commonly found in riverbeds, glacial till, and coastal beaches globally. Collectors look for unique vein patterns.
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