sedimentary
Chert (with Manganese Oxide coating)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs); Color: Brownish-yellow base with black coating; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Brownish-yellow base with black coating
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Brownish-yellow base with black coating; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. The black coating is likely a secondary deposit of manganese oxide (desert varnish) formed by bacterial action or chemical weathering.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for river rock landscaping, road ballast, and lapidary tumbling.
Geological facts
Chert is incredibly durable and chemically stable, allowing it to survive long-distance transport in rivers and streams, which often results in the rounded, water-worn shape seen here.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy texture, and shell-like (conchoidal) fracture pattern. Commonly found in stream beds, beaches, and limestone outcrops.
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