Chert (Black Chert)

Physical Properties

Hardness: 7 on Mohs scale; Color: Black, dark gray, and tan; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed by the precipitation of silica-rich fluids either as a primary chemical precipitate or as a replacement of limestone. This specimen appears to have formed in a marine environment where silica from diatoms or radiolarians accumulated.

Uses & Applications

Historically used for stone tools and weapons (flint-knapping). Modern uses includes road surfacing, abrasives, and as decorative landscaping stone.

Geological Facts

Depending on its color and origin, it is often called flint, jasper, or onyx. It is so hard that it can strike sparks from steel, which led to its use in flintlock firearms.

Field Identification & Locations

Identify by its characteristic conchoidal (shell-like) fracture edges, waxy feel, and ability to scratch glass. Commonly found in limestone beds as nodules or lenses.

Identified on: 5/1/2026

Mode: Standard