Chert (Nodule)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, brown; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation within limestone or chalk beds. Often found in Phanerozoic sedimentary sequences.

Uses & Applications

Historically used for stone tools (arrowheads). Modernly used as an abrasive, in road construction (aggregate), and for landscaping. High-quality varieties are used in lapidary work.

Geological Facts

Chert is so hard it can scratch steel and produce sparks when struck against iron, which made it vital for early 'flintlock' firearms. It is virtually immune to chemical weathering.

Field Identification & Locations

Identify in the field by its extreme hardness, waxy texture, and sharp, shell-like (conchoidal) edges when broken. It often occurs as rounded nodules or 'lumps' protruding from softer limestone walls.

Identified on: 4/26/2026

Mode: Standard