Chert (Jasper/Flint variety)

Physical Properties

Hardness: 7 on Mohs scale; Color: typically dark gray, brown, or earthy green with reddish staining; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater or marine environments. It is often found as nodules or layers within limestone and chalk beds.

Uses & Applications

Historically used for making stone tools (arrowheads, knives) due to its sharp edges; used in construction aggregate, and occasionally in lapidary work for jewelry or polished decorative stones.

Geological Facts

Chert was one of the most important minerals for early humans because its conchoidal fracture allows it to be knapped into extremely sharp tools. It is also famous for its ability to create sparks when struck against steel, which made its variety 'flint' the standard for fire-starting and early firearms.

Field Identification & Locations

Identify in the field by its smooth, waxy surface and the presence of shell-like (conchoidal) fractures where it has been broken. It is harder than steel and will scratch glass easily. Commonly found in riverbeds, limestone outcrops, and coastal cliffs.

Identified on: 4/18/2026

Mode: Standard