Chert (Nodule)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Tan, beige, off-white; Luster: Dull to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Formation & Geological History

Formed by the precipitation of silica from groundwater within limestone or chalk beds, or by the accumulation of siliceous skeletal remains on the seafloor. This specimen shows typical water-worn rounding.

Uses & Applications

Primarily used in construction as road base or aggregate; historically used for stone tools due to its sharp edges when fractured. Some colorful varieties are used for lapidary work.

Geological Facts

Chert is extremely durable and chemically stable. Ancient civilizations used chert and its variety, flint, for making arrowheads and spear points because it breaks with a predictably sharp edge.

Field Identification & Locations

Identify by its hardness (it will scratch glass), its smooth or conchoidal fracture surface upon breaking, and its waxy luster. Commonly found in riverbeds or eroding out of limestone outcrops.

Identified on: 5/12/2026

Mode: Standard