sedimentary
Chert (Green River Chert)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Olive green to dull tan; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Olive green to dull tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Olive green to dull tan; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed via the precipitation of silica from groundwater or the accumulation of silica-rich organic debris (like radiolarians) in marine or lake environments. This specimen appears to be a water-worn river pebble.
Uses & applications
Historically used for stone tools (flintknapping) due to its sharp edges; used today as decorative gravel, in landscaping, and as a low-cost lapidary material.
Geological facts
The red spots visible on this rock are likely Clover Mites (Bryobia praetiosa) or Velvet Mites, rather than part of the mineralogy. Chert is so hard it can scratch steel and was used to start fires before matches.
Field identification & locations
Identify in the field by its waxy texture, curved (conchoidal) fractures similar to glass, and its ability to scratch glass or steel. Found globally in limestone beds or as river cobbles.
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