Chert (Quartz Variety)

Physical Properties

Hardness: 7 (Mohs scale); Color: Creamy white to light tan; Luster: Waxy to dull; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed from the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) on the ocean floor. Most deposits date from the Precambrian to the Cretaceous periods.

Uses & Applications

Historically used for stone tools (arrowheads/scrapers) due to its sharp edges; used in modern times for road surfacing, as grinding media, and sometimes in landscaping.

Geological Facts

Chert is so hard it can strike sparks against steel, making it useful for prehistoric fire-starting. It is essentially a very fine-grained version of quartz.

Field Identification & Locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Found globally in limestone and chalk beds.

Identified on: 4/18/2026

Mode: Standard