Chert (often called Flint or River Stone)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, grey, and iron-stained orange; Luster: Dull to waxy; Crystal Structure: Microcrystalline/cryptocrystalline (no visible crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation in marine or groundwater environments. These specimens appear water-worn, likely from a fluvial (river) environment.

Uses & Applications

Historically used for stone tools and arrowheads due to its sharp conchoidal fracture. Currently used as construction aggregate, decorative landscaping stone, and occasionally for fire-starting (flint).

Geological Facts

Chert is so hard it can scratch steel. When struck against steel, it can create sparks, which is why the variety known as 'flint' was essential for early firearms and survival. It is chemically identical to quartz but has a different microscopic structure.

Field Identification & Locations

Identify in the field by its extreme hardness, smooth texture, and 'shell-like' curved fractures (conchoidal). It is commonly found in riverbeds, gravel pits, and as nodules within limestone or chalk layers.

Identified on: 4/20/2026

Mode: Standard