Chert (containing Calcite or Quartz inclusions)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, and white; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (such as diatoms or radiolarians) or chemical precipitation in marine or lacustrine environments. Often found as nodules in limestone or chalk beds.

Uses & Applications

Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracture. Modern uses include road surfacing, decorative garden stones, and jewelry making (especially when patterned).

Geological Facts

Chert is so hard it can scratch glass and sparks when struck against steel. It is chemically identical to quartz but has a different internal microscopic structure.

Field Identification & Locations

Identify in the field by its waxy texture, smooth conchoidal (shell-like) fracture edges, and hardness. Often found in riverbeds or coastal areas where limestone has eroded away.

Identified on: 4/22/2026

Mode: Standard