sedimentary
Chert with Quartz/Calcite Vein
Microcrystalline Silica (SiO2)
Hardness: 7 (Mohs scale); Color: Dark grey to black matrix with a yellowish-white vein; Luster: Waxy to dull (glassy when wet); Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Dark grey to black matrix with a yellowish-white vein
- Luster
- Waxy to dull (glassy when wet)
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark grey to black matrix with a yellowish-white vein; Luster: Waxy to dull (glassy when wet); Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. The vein formed later through hydrothermal activity where mineral-rich water filled a fracture.
Uses & applications
Used historically for stone tools and arrowheads due to its sharp edges; currently used for road surfacing, decorative landscaping, and as a collector's specimen.
Geological facts
Black chert is often referred to as 'flint'. The 'vein' seen here is a signature of tectonic stress where the brittle rock cracked and was later healed by secondary mineral fluids.
Field identification & locations
Identify by its extreme hardness (scratches glass), smooth waxy feel, and characteristic shell-like (conchoidal) fracture pattern. Commonly found in riverbeds and coastal areas.
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