Chert (and Flint)

Physical Properties

Hardness: 7 (Mohs scale); Color: Dark grey, black, brown, or tan; Luster: Waxy to dull; Crystal Structure: Microcrystalline (Cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within sedimentary beds like limestone. Found in strata dating back to the Precambrian era.

Uses & Applications

Historically used for stone tools and starting fires (flint); currently used in road construction, as an abrasive, and sometimes in jewelry (as jasper or agate variants).

Geological Facts

Chert was one of the most important materials for early humans because its conchoidal fracture produces extremely sharp edges suitable for arrowheads and knives. It is chemically very stable and resistant to weathering.

Field Identification & Locations

Identify in the field by its waxy texture, extreme hardness (can scratch glass), and distinctive shell-like (conchoidal) curved fracture lines when broken. Commonly found in riverbeds or as nodules within limestone outcrops.

Identified on: 4/19/2026

Mode: Standard