sedimentary
Chert (with Quartz veins)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Dark brown (liver-colored) with white veins; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Dark brown (liver-colored) with white veins
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Dark brown (liver-colored) with white veins; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of siliceous remains of marine organisms (like diatoms or radiolaria) or by chemical precipitation. The white veins are secondary quartz infilling fractures caused by tectonic stress.
Uses & applications
Used historically for stone tools due to its sharp edges; currently used as decorative gravel, in lapidary work as 'river stones', and for fire-striking.
Geological facts
Chert is so hard it can scratch steel. It is chemically identical to flint, but flint is usually found in chalk or marly limestone, whereas chert is found in many types of limestone. The white 'veins' are called 'septaria' when they form specific patterns.
Field identification & locations
Identify by its waxy appearance and smooth, curved (conchoidal) fractures. The white veins will not react to acid, distinguishing them from calcite. Common in riverbeds and glacial tills.
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