Chert (Flint Nodules)

Physical Properties

Hardness: 7 on Mohs scale; Color: Blue-grey interior with a tan/cream weathered cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & Geological History

Formed by the precipitation of silica in sedimentary rock layers, often as nodules within limestone or chalk. Many deposits date from the Cretaceous period, formed from the remains of silica-rich organisms like sponges or diatoms.

Uses & Applications

Historically used for stone tools and weapons (knapping); currently used in road construction, as a filler in ceramics, and occasionally as a semi-precious gemstone (Jasper/Agate varieties).

Geological Facts

Because of its conchoidal fracture, it breaks into pieces with very sharp edges, leading to its extensive use by prehistoric humans for arrowheads and scrapers. Blue-grey varieties are specifically prized in areas like the UK and Scandinavia.

Field Identification & Locations

Identified in the field by its distinctive 'cortex' (a rough, weathered outer skin) and glass-like, shell-shaped fracture patterns when broken. Commonly found in chalk downs or as river/beach gravel.

Identified on: 4/27/2026

Mode: Standard