sedimentary
Chert
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: brown, tan, gray; Luster: waxy to dull; Crystal structure: Trigonal (microscopic crystals); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.6
- Hardness
- 7 on Mohs scale
- Color
- brown, tan, gray
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: brown, tan, gray; Luster: waxy to dull; Crystal structure: Trigonal (microscopic crystals); Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.6
Formation & geological history
Formed by the precipitation of silica in sedimentary environments, often replacing organic matter or limestone; can be biogenic or chemical in origin. Ages vary widely from Precambrian to Cenozoic.
Uses & applications
Used historically for stone tools due to sharp edges; currently used for road base, decorative landscaping, and occasionally jewelry (as jasper).
Geological facts
Chert is essentially the same material as flint, though flint is generally darker. It is so tough that it was the primary choice for arrowheads and scrapers by prehistoric humans.
Field identification & locations
Identify by its conchoidal (shell-like) fracture and high hardness—it will scratch glass and even steel. Commonly found in limestone or chalk beds.
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Brown Sandstone (River Rock)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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