sedimentary
Chert (Nodular Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, and creamy white; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, and creamy white
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, and creamy white; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed in marine environments through the precipitation of silica, often replacing carbonate material in limestone or chalk. These nodules can date from the Proterozoic to the present day.
Uses & applications
Historically used for stone tools (knapping) and sparking fire. Currently used as a road aggregate, in landscaping, and as a decorative stone in jewelry (when banded).
Geological facts
This specimen appears to be a nodular chert with a weathered outer 'cortex' (white) and a dark silica interior. It is the same material used by prehistoric humans to create arrowheads and axes.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy luster on fresh breaks, and smooth, curved conchoidal fractures. Commonly found in riverbeds, glacial till, and embedded in limestone outcrops.
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