sedimentary

Chert (often called Flint when dark)

Chert (Microcrystalline or Cryptocrystalline Silica, SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with waxy luster; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, tan, or brown with waxy luster

Identified February 14, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with waxy luster; Crystal Structure: Trigonal/Microcrystalline; Cleavage: None (exhibits prominent conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. It can date back to the Precambrian, though common in Paleozoic and Mesozoic strata.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Industrially used as an abrasive, in road construction (aggregate), and occasionally polished as semi-precious gemstones or decorative landscape stone.

Geological facts

Chert was the high-tech material of the Stone Age due to its ability to be chipped into razor-sharp edges. It is so tough that it can spark when struck against steel, a property that led to the development of flintlock firearms.

Field identification & locations

Identify in the field by its smooth, waxy surface and curved, shell-like (conchoidal) fractures. It will easily scratch glass. Common in riverbeds, limestone outcrops, and coastal cliffs globally.