Chert (Brecciated Chert)

Physical Properties

Hardness: 7 (Mohs scale), Color: Tan to brown with dark grey/black inclusions, Luster: Waxy to dull, Crystal structure: Cryptocrystalline (Microcrystalline Quartz), Cleavage: None (conchoidal fracture), Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich microfossils or precipitation from silica-rich fluids in sedimentary basins. This specific specimen appears water-worn (a 'river rock' or 'beach pebble') and shows signs of brecciation, where original rock fragments were broken and re-cemented by silica.

Uses & Applications

Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for decorative landscaping, gravel, and occasionally tumbled for jewelry or pocket stones.

Geological Facts

Chert is one of the most durable sedimentary rocks. It was the primary material used by prehistoric humans to create fire-starting tools and weaponry. The dark patches in this specimen are likely concentrations of organic matter or different mineral oxides.

Field Identification & Locations

Identify in the field by its extreme hardness (will scratch glass and steel), waxy texture, and smooth, rounded surface when found in riverbeds. It is common worldwide in limestone or dolostone layers.

Identified on: 4/26/2026

Mode: Standard