sedimentary
Chert / Chalcedony Nodules
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, light brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 6
- Color
- Gray, tan, light brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, light brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich fluids in sedimentary rock layers, often limestone or chalk. The silica typically originates from the skeletal remains of marine organisms like sponges and radiolarians, which dissolve and precipitate as nodules over millions of years.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when broken. Also used in construction as an aggregate and for decorative landscaping.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture creates edges sharper than a surgical scalpel. It is highly resistant to chemical weathering.
Field identification & locations
Identify in the field by its significant hardness (it will scratch glass) and its smooth, waxy feel in comparison to surrounding limestone. Look for rounded, lumpy shapes with a white or tan weathered rind.
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