sedimentary
Chert Nodules (Limonitic Chert)
Cryptocrystalline Quartz (SiO2) with Hydrated Iron Oxides
Hardness: 6.5-7 (Mohs); Color: Brown, tan, earthy yellow-brown; Luster: Dull to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Brown, tan, earthy yellow-brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Brown, tan, earthy yellow-brown; Luster: Dull to waxy; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.
Formation & geological history
Formed via the accumulation of silica-rich fluids within sedimentary host rocks like limestone or chalk. The brown color is due to limonitic staining from iron oxides. Often found in Paleozoic to Cenozoic strata.
Uses & applications
Historically used for making stone tools (flint knapping) due to its sharp edges. Modernly used as road aggregate, architectural stone, and occasionally for tumbling as decorative pebbles.
Geological facts
This specimen appears to be 'Tripstow stone' or a similar iron-stained silicate nodule. Chert is so durable that it often remains behind after the surrounding host rock has eroded away.
Field identification & locations
Identified by its extreme hardness (will scratch glass), opaque nature, and dull, earthy exterior. Commonly found in creek beds and fields where sedimentary rock has weathered.
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