sedimentary

Chert

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 Mohs; Color: Gray, green, tan; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs
Color: Gray, green, tan
Luster: Waxy to vitreous

Identified July 10, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Gray, green, tan; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formation occurs as nodules or layers in sedimentary rocks (like limestone) through the precipitation of silica-rich fluids or the accumulation of siliceous skeletal remains of marine organisms.

Uses & applications

Historically used for stone tools and starting fires; currently used in construction aggregate, road surfacing, and as a semi-precious gemstone when colorful (like Jasper).

Geological facts

Chert has been used by humans for hundreds of thousands of years to create sharp blades and arrowheads because it breaks with a predictable, razor-sharp conchoidal fracture.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and distinctive shell-like (conchoidal) fracture patterns on broken edges.