sedimentary
Chert / Quartz Pebble
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 (Mohs scale), Color: Creamy white with brownish iron staining, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Creamy white with brownish iron staining, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary layers. This specific specimen shows signs of fluvial or coastal tumbling, becoming smooth over millions of years.
Uses & applications
Commonly used as construction aggregate, road base, and in landscaping. Historically used for making stone tools due to its conchoidal fracture which creates sharp edges.
Geological facts
Chert is so durable that it often survives as 'river rock' long after the surrounding limestone or shale has eroded away. The brown markings on this specimen are likely limonite or iron oxide staining from the soil/water environment.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, waxy texture compared to granite or limestone. Look for 'chatter marks' or small crescent-shaped impact scars typical of water-transported pebbles.
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