sedimentary
Chert (and Limestone)
Silicon Dioxide (SiO2) in Calcium Carbonate (CaCO3)
Hardness: 7 (chert) to 3 (limestone); Color: buff, tan, white, and earthy yellow; Luster: dull to waxy; Texture: fine-grained/cryptocrystalline; Fractures: conchoidal (shell-like) in the chert portions.
- Hardness
- 7 (chert) to 3 (limestone)
- Color
- buff, tan, white, and earthy yellow
- Luster
- dull to waxy
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Physical properties
Hardness: 7 (chert) to 3 (limestone); Color: buff, tan, white, and earthy yellow; Luster: dull to waxy; Texture: fine-grained/cryptocrystalline; Fractures: conchoidal (shell-like) in the chert portions.
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation within marine limestone beds. Common in sedimentary basins worldwide, particularly in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools/arrowheads. Today used primarily as road base, construction aggregate, and occasionally as a decorative landscape stone.
Geological facts
Chert is so hard it can spark when struck against steel. It is chemically identical to quartz but has a different micro-crystalline structure. This specific specimen appears to be a weathered fragment with some iron-oxide staining.
Field identification & locations
Identify by its hardness (chert will scratch glass) and its association with softer limestone that often fizzes in weak acid. Generally found in gravel pits, riverbeds, or limestone outcrops.
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