sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Varying shades of gray, brown, and tan with white veins; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Varying shades of gray, brown, and tan with white veins
Luster: Waxy to dull

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in marine environments. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons (arrowheads/knives) and as a spark generator (flint). Currently used as a hard aggregate in road construction and occasionally in craft jewelry.

Geological facts

Chert is so hard it can scratch steel. Because it breaks with a conchoidal fracture (smooth, curved surfaces), it can produce edges sharper than a surgical scalpel, which is why it was the primary material for prehistoric 'flint knapping'.

Field identification & locations

Identify in the field by its very smooth, glass-like fracture surfaces and extreme hardness. It is common in riverbeds and limestone outcrops globally. Look for 'waxy' texture compared to surrounding rocks.