Chert (Flint nodule)

Physical Properties

Hardness: 7 (Mohs scale); Color: tan, grey, brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits conchoidal fracture; Specific gravity: 2.65

Formation & Geological History

Formed through the precipitation of silica from seawater or groundwater within limestone or chalk formations. These nodules typically date from the Cretaceous period to the present, often beginning as silica-rich organic remains.

Uses & Applications

Used historically for stone tools and projectile points due to its sharp edges; used today as decorative gravel, in road construction, and for fire-starting strikers.

Geological Facts

Chert was one of the most important minerals for early humans, used to create flint tools. Its sharp conchoidal edges can be sharper than a surgical scalpel. It is chemically identical to quartz but contains microscopically small crystals.

Field Identification & Locations

Identify in the field by its smooth, waxy texture and shell-like (conchoidal) fracture pattern. It will easily scratch glass and will not react with acid like the limestone it is often found in.

Identified on: 4/15/2026

Mode: Standard