sedimentary
Chert (also known as Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Brown, grey, tan; Luster: Waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); SG: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Brown, grey, tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Brown, grey, tan; Luster: Waxy to dull; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); SG: 2.65
Formation & geological history
Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or by chemical precipitation of silica in groundwater within sedimentary host rocks. Common in ancient limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons (arrowheads/knives) due to sharp edges; currently used as a road surfacing material, in construction aggregate, and occasionally as decorative landscaping stone.
Geological facts
Flint is a variety of chert that has been used by humans for millions of years to start fires and make tools. It produces sparks when struck against steel. Many nodules contain internal cavities or fossils.
Field identification & locations
Identify by its extremely smooth, shell-like (conchoidal) fracture surfaces and its ability to easily scratch glass. Found in nodules or layers within sedimentary rock units like limestone.
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