Chert (Nodule)

Physical Properties

Hardness: 7 (Mohs scale); Color: Variegated white, tan, and reddish-brown (iron-stained); Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the replacement of limestone/chalk by silica-rich fluids or the accumulation of siliceous organisms like diatoms and radiolarians on the ocean floor. Most nodules date from the Cretaceous period to the Paleozoic.

Uses & Applications

Historically used for stone tools (flintknapping); currently used in road construction as aggregate, ornamental landscaping, and occasionally as a semi-precious gemstone (Jasper/Agate varieties).

Geological Facts

Chert is so durable that it often remains as 'river rocks' long after the surrounding limestone has eroded away. It is one of the most common materials used by prehistoric humans to create sharp-edged tools like arrowheads.

Field Identification & Locations

Identify in the field by its superior hardness (will scratch glass), conchoidal (shell-like) fracture pattern when broken, and waxy texture. Commonly found in limestone outcrops, river beds, and gravel pits.

Identified on: 4/21/2026

Mode: Standard