Chert (often called Flint or Jasper)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, black, white, and reddish-brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or by chemical precipitation from hydrothermal fluids in limestone or chalk beds. Most specimens date from the Phanerozoic eon.

Uses & Applications

Used historically for stone tools and weapons (arrowheads). Modern uses include road surfacing, decorative garden stones, and occasionally as semi-precious gemstones in jewelry (Jasper).

Geological Facts

Chert was one of the most important technological materials for early humans because its conchoidal fracture creates extremely sharp edges; it is also capable of creating sparks when struck against steel.

Field Identification & Locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and smooth, shell-like (conchoidal) fractures. Common in riverbeds and limestone outcrops globally.

Identified on: 4/19/2026

Mode: Standard