Chert (containing Flint and Chalcedony)

Physical Properties

Hardness: 7 (Mohs scale), Color: White, grey, brownish-tan with blue-grey inclusions, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & Geological History

Formed by the precipitation of silica in marine or groundwater environments. This specimen likely formed through the replacement of limestone or chalk by silica-rich fluids. It occurs in nodules or stratified layers throughout various geological ages, frequently Paleozoic and Mesozoic.

Uses & Applications

Historically used for stone tools and fire-starting. Today used as an abrasive, in road construction, and higher quality translucent specimens are used for jewelry cabochons or decorative carvings.

Geological Facts

Chert was one of the most vital minerals for early human civilizations because of its ability to fracture into extremely sharp edges (conchoidal fracture). It is chemically identical to quartz but contains impurities and has an microscopic grain structure.

Field Identification & Locations

Identify in the field by its hardness (will scratch glass), waxy feel on fresh surfaces, and smooth, shell-like (conchoidal) breakage patterns. Common in limestone deposits and as river-worn pebbles. For collectors, look for unique colors or translucency.

Identified on: 4/19/2026

Mode: Standard