Chert (Chalky variety)

Physical Properties

Hardness: 7 on Mohs scale; Color: Chalky white exterior with grayish-tan interior; Luster: Dull to waxy; Structure: Microcrystalline; 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 ground fluids within limestone or chalk beds. Often Cretaceous in age when found in chalk deposits.

Uses & Applications

Historically used for stone tools (flintknapping). Modernly used in road construction, as an abrasive, or for decorative landscaping. Some high-quality translucent varieties are used for jewelry.

Geological Facts

Chert is so hard it can create sparks when struck against steel. It is chemically very stable and resistant to weathering, which is why it often remains as smooth pebbles in riverbeds or beaches.

Field Identification & Locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found in river gravels, limestone quarries, and coastal beaches.

Identified on: 5/4/2026

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