sedimentary
Chert (with Quartz/Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Variegated brown, grey, white, and reddish-orange; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Variegated brown, grey, white, and reddish-orange
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Variegated brown, grey, white, and reddish-orange; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich fluids in limestone or through the accumulation of siliceous biological remains (like diatoms or radiolarians). Common in sedimentary basins and often found as nodules or layers.
Uses & applications
Historically used for making stone tools (arrowheads). Modernly used in construction aggregate, as landscaping stone, or as decorative specimens for collectors when polished.
Geological facts
Chert is so hard it can strike sparks against steel. The reddish tints in this specific specimen likely come from iron oxide (hematite) impurities staining the silica.
Field identification & locations
Identify by its waxy appearance and conchoidal (shell-like) fracture patterns when broken. Look for it in limestone or chalk outcrops. Collectors should check for translucency at the edges.
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