sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale, Color: Brown, tan, yellowish, or grey, Luster: Vitreous to dull, Crystal structure: Microcrystalline/cryptocrystalline, Cleavage: Conchoidal fracture, Specific Gravity: 2.60-2.65
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Physical properties
Hardness: 7 on the Mohs scale, Color: Brown, tan, yellowish, or grey, Luster: Vitreous to dull, Crystal structure: Microcrystalline/cryptocrystalline, Cleavage: Conchoidal fracture, Specific Gravity: 2.60-2.65
Formation & geological history
Formed in marine environments through the chemical precipitation of silica from seawater or the replacement of limestone by silica-rich solutions, often occurring in nodules within chalk or limestone formations.
Uses & applications
Historically used by early humans for making stone tools such as arrowheads, spear points, and scrapers due to its conchoidal fracture which produces sharp edges.
Geological facts
Chert of certain colors is known by other names such as flint (dark/black) or jasper (red/brown). It produces a spark when struck against steel, which made it valuable for fire-starting.
Field identification & locations
Can be identified in the field by its hard, dense nature, smooth or waxy texture, and characteristic sharp conchoidal fracture. Commonly found in sedimentary rock layers worldwide.
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