sedimentary
Chert (Quartz Variety)
Chert (mostly SiO2 / Silicon Dioxide)
Hardness: 7 (Mohs scale); Color: Creamy white to pale tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: ~2.65
- Hardness
- 7 (Mohs scale)
- Color
- Creamy white to pale tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Creamy white to pale tan; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: ~2.65
Formation & geological history
Forms primarily through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation in limestone or chalk beds. This specimen appears weathered and water-worn, potentially from a riverbed environment.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to conchoidal fracture. Modernly used as road base, decorative landscaping rock, and occasionally for lapidary work.
Geological facts
Chert is so hard it can create sparks when struck against steel. It is a dense, hard sedimentary rock that is chemically stable and very resistant to weathering.
Field identification & locations
Identify in the field by its hardness (will scratch glass), waxy luster, and its characteristic curved, shell-like (conchoidal) fracture pattern when broken. It is commonly found in gravel pits and as nodules within limestone outcrops.
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