sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, white, and brown banding; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Hardness: 7 (Mohs scale)
Color: Tan, white, and brown banding
Luster: Waxy to vitreous

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Tan, white, and brown banding; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Formation & geological history

Formed primarily through the replacement of limestone or carbonate sediments by silica-rich fluids. It often occurs as nodules or layers within sedimentary rock sequences, sometimes originating from the accumulation of siliceous skeletal remains of marine organisms.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used as decorative ground cover, in road construction as aggregate, and by lapidary hobbyists for polishing.

Geological facts

Chert has been essential to human development; the sharp edges produced by its conchoidal fracture allowed early humans to create the first sophisticated hunting tools and scrapers. Some chert contains well-preserved microfossils.

Field identification & locations

Identified by its extreme hardness (will scratch glass), waxy appearance, and characteristic shell-like (conchoidal) fracture patterns when broken. Common in limestone outcrops and riverbeds.