sedimentary
Chert (Agatized/Banded)
Chert (Microcrystalline Silicon Dioxide, SiO2)
Hardness: 7 Mohs scale; Color: Tan, cream, and gray with concentric banding; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 Mohs scale
- Color
- Tan, cream, and gray with concentric banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs scale; Color: Tan, cream, and gray with concentric banding; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or chemical precipitation in sedimentary environments. The banding suggests rhythmic deposition or later silica replacement within a host rock over millions of years.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; currently used for landscaping, aggregate, and as a semi-precious gemstone when polished.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is one of the most common materials used by early humans for lithic technology.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy feel, and smooth, curved (conchoidal) fracture surfaces. Commonly found in limestone or chalk beds as nodules or layers.
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