sedimentary

Chert (Nodule)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 (Mohs scale); Color: Brown, tan, or gray; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Brown, tan, or gray
Luster: Dull to waxy

Identified April 1, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Brown, tan, or gray; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from silica-rich groundwater within limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads). Modernly used as road aggregate, in construction, and occasionally for lapidary work.

Geological facts

Chert is so hard it can scratch glass and steel. When struck against steel, it can produce sparks, leading to its historical use in flintlock firearms (as the 'flint' variety).

Field identification & locations

Identify by its extreme hardness, smooth conchoidal (curved) fracture surfaces when broken, and lack of visible grains. Commonly found as nodules in limestone outcrops or as river-worn pebbles.