sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Grayish-tan to light green; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Grayish-tan to light green
Luster: Waxy to dull

Identified July 3, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Grayish-tan to light green; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: Conchoidal fracture; Specific gravity: 2.65.

Formation & geological history

Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids in sedimentary basins. Primarily Paleozoic to Mesozoic in age.

Uses & applications

Historically used for stone tools (arrowheads, scrapers); currently used in road construction, as an abrasive, and sometimes as a landscape rock.

Geological facts

Chert is so hard it can strike sparks from steel, which is why it was famously used as flint in fire-starting and early firearms. It is chemically identical to quartz but has a different internal structure.

Field identification & locations

Identified in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone or chalk beds.