Chert (Nodule)

Physical Properties

Hardness: 7 (Mohs scale); Color: tan, brownish-yellow, dull orange; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8.

Formation & Geological History

Formed through the accumulation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. Many nodules form during diagenesis as silica replaces calcium carbonate over millions of years.

Uses & Applications

Historically used for stone tools and starting fires (flint); currently used in road construction, as an abrasive, and occasionally for lapidary work if highly colorful.

Geological Facts

Chert is so hard it can scratch steel. When it breaks, it produces extremely sharp edges, which is why prehistoric humans used it for arrowheads and scrapers. The yellowish variety is often colored by limonite or iron oxide impurities.

Field Identification & Locations

Identify by its waxy texture, extreme hardness, and 'shell-like' conchoidal fractures where it has been chipped. Commonly found in riverbeds, glacial till, or weathered out of limestone cliffs.

Identified on: 4/13/2026

Mode: Standard