sedimentary
Chert
Sedimentary cryptocrystalline quartz (SiO2)
Hardness: 7 on Mohs scale. Color: Varies widely including white, grey, brown, yellowish, reddish, or black, often with a weathered outer rind (patina). Luster: Vitreous to dull. Crystal structure: Microcrystalline or cryptocrystalline quartz (chalcedony). Cleavage: None, exhibits conchoidal fracture.…
- Hardness
- 7 on Mohs scale
- Luster
- Vitreous to dull
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Physical properties
Hardness: 7 on Mohs scale. Color: Varies widely including white, grey, brown, yellowish, reddish, or black, often with a weathered outer rind (patina). Luster: Vitreous to dull. Crystal structure: Microcrystalline or cryptocrystalline quartz (chalcedony). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.60 to 2.65.
Formation & geological history
Forms primarily as a chemical sedimentary rock through the replacement of limestone or chalk by silica-rich groundwater, or through the accumulation of siliceous skeletal remains of microscopic organisms like radiolarians and diatoms. Geological age ranges from Precambrian to modern times.
Uses & applications
Historically used extensively by prehistoric humans for making stone tools (such as arrowheads, scrapers, and blades) due to its conchoidal fracture producing sharp edges. Occasionally used in modern decorative landscaping or as an aggregate.
Geological facts
Chert is extremely durable and weather-resistant, which is why ancient stone tools made from it survive exceptionally well in the archaeological record. Varieties of chert include flint, jasper, and agate depending on color and impurities.
Field identification & locations
Can be identified in the field by its extreme hardness (scratches glass easily), smooth or waxy texture, and characteristic conchoidal fracture with sharp edges. Commonly found as nodules or layers within limestone and chalk formations worldwide.
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