sedimentary

Chert (Arrowhead variety)

Chert (SiO2)

Hardness: 6.5 - 7 (Mohs scale); Color: Dark brown to black (flint-like); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7.

Hardness: 6
Color: Dark brown to black (flint-like)
Luster: Waxy to dull

Identified January 2, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 6.5 - 7 (Mohs scale); Color: Dark brown to black (flint-like); Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical replacement of limestone/dolomite. It is common in Phanerozoic sedimentary sequences.

Uses & applications

Historically used for stone tools and weapons due to its ability to hold a sharp edge. Industrially used as an abrasive or in construction aggregate. Highly valued by archaeological collectors.

Geological facts

This specimen is a 'projectile point,' an artifact crafted through a process called knapping. Chert was the 'steel of the ancient world' because its conchoidal fracture creates edges sharper than a modern steel scalpel.

Field identification & locations

Identify in the field by looking for 'scalloped' or shell-like fracture patterns (conchoidal) and a waxy texture. Often found as nodules within limestone outcrops or as weathered pebbles in stream beds.