sedimentary
Chert with Iron Staining
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Dark gray to black matrix with reddish-brown (limonite/hematite) iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark gray to black matrix with reddish-brown (limonite/hematite) iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed as a chemical precipitate from silica-rich fluids or the accumulation of siliceous organic remains (radiolarians/diatoms) in marine environments. The iron staining occurs via oxidation in groundwater.
Uses & applications
Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used for road ballast, abrasives, or as lapidary rough for polishing.
Geological facts
Chert was one of the first materials utilized by early humans for fire-starting and tool-making. It is so tough it will easily scratch glass and even steel.
Field identification & locations
Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its extreme hardness. Common in limestone formations and as river pebbles.
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