sedimentary
Chert (specifically Chert Nodule/Flint)
Chert (Silicon Dioxide, SiO2)
Hardness: 7 on the Mohs scale. Color: Gray, brown, and reddish-orange due to iron oxides. Luster: Dull to waxy. Crystal structure: Cryptocrystalline (microscopic quartz). Cleavage: None (conchoidal/shell-like fracture). Specific gravity: 2.5-2.8.
- Hardness
- 7 on the Mohs scale
- Color
- Gray, brown, and reddish-orange due to iron oxides
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on the Mohs scale. Color: Gray, brown, and reddish-orange due to iron oxides. Luster: Dull to waxy. Crystal structure: Cryptocrystalline (microscopic quartz). Cleavage: None (conchoidal/shell-like fracture). Specific gravity: 2.5-2.8.
Formation & geological history
Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) within limestone or chalk beds. This specific specimen appears to be a weathered nodule, potentially dating from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads/knives) due to its sharp conchoidal fracture. Industrially used as an abrasive, in road construction as gravel, and in some decorative lapidary jewelry.
Geological facts
Chert is so hard it can scratch glass and steel. High-quality dark chert is often called 'flint' and was essential to early humans for making fire and weapons. It is chemically very stable and resistant to weathering.
Field identification & locations
Identify by its waxy texture, extreme hardness, and 'dish-shaped' conchoidal fractures where pieces have chipped off. Common in limestone outcrops, riverbeds, and glacial till. Collectors look for pieces with unique banding or translucency.
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