sedimentary
Chert (with Quartz vein/coating)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray with white/cream mineral coating; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray with white/cream mineral coating
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray with white/cream mineral coating; Luster: Dull to waxy; Structure: Microcrystalline; 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 white veins are typically secondary quartz or calcite precipitated from groundwater in cracks.
Uses & applications
Historically used for making stone tools (arrowheads) due to conchoidal fracturing. Modernly used as construction aggregate, road surfacing, or as a decorative landscaping stone.
Geological facts
Chert is so hard it can strike sparks against steel. It is one of the most common materials used by prehistoric humans for tool-making worldwide.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel pocket knife) and the way it breaks into smooth, curved surfaces. Commonly found in limestone beds or as rounded pebbles in riverbeds and beaches.
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