sedimentary
Chert (Iron-stained)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Rusty brown, tan, and gray; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Rusty brown, tan, and gray
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 Mohs scale; Color: Rusty brown, tan, and gray; Luster: Dull to waxy; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica from groundwater or within marine sediments. The iron-staining (limonite/hematite) suggests exposure to oxygenated water and weathering over geological time.
Uses & applications
Historically used for stone tools (arrowheads). Modern uses include road aggregate, decorative landscaping, and as a collector's specimen for lapidary work.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, similar to flint. It is one of the most durable sedimentary rocks.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its inability to be scratched by a steel knife. Commonly found in limestone beds or as weathered nodules in stream beds.
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