sedimentary
Chert (Flint)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 6.5-7 Mohs; Color: Gray, light blue-gray, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Gray, light blue-gray, tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Gray, light blue-gray, tan; 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 silica-rich microfossils (like radiolarians or diatoms) or by precipitation from silica-rich fluids in sedimentary environments such as limestone or chalk beds. Often Cretaceous to Jurassic in age.
Uses & applications
Historical use for stone tools and igniting fire; modern use in road construction, landscaping, and occasionally as a gemstone in lapidary work.
Geological facts
Chert is so hard it can scratch steel. When broken, it produces a very sharp, shell-like (conchoidal) edge, which is why it was the primary material for prehistoric arrowheads and spear tips.
Field identification & locations
Look for a smooth, waxy texture and curved, glass-like fracture surfaces. It is commonly found as nodules in limestone or as rounded pebbles in riverbeds and on beaches. It will not react with acid, distinguishing it from limestone.
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