sedimentary
Chert (with Quartz Veining)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Smoky grey to black with white/tan veins, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Fracture: Conchoidal
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Physical properties
Hardness: 7 (Mohs scale), Color: Smoky grey to black with white/tan veins, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Fracture: Conchoidal
Formation & geological history
Formed as a result of the accumulation of siliceous remains of marine organisms or through chemical precipitation in sedimentary basins; primarily Cretaceous to Paleogene in the Central Texas region.
Uses & applications
Historically used for stone tools (knapping); currently used as architectural aggregate, road base, and for ornamental polishing or tumbling by collectors.
Geological facts
This specimen shows 'veining' where tectonic stress fractured the original rock, and secondary minerals (likely quartz) precipitated into the cracks later. It is highly resistant to weathering.
Field identification & locations
Commonly found in the Edwards Plateau and surrounding Texas river beds. Look for its smooth, waxy texture and sharp, shell-like edges when broken. It is harder than steel and will scratch glass easily.
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