sedimentary
Chert / Chalcedony Nodules
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Gray, blue-gray, brown, tan; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 6
- Color
- Gray, blue-gray, brown, tan
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Gray, blue-gray, brown, tan; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline); Cleavage: None (Conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich fluids within sedimentary rock layers, often limestone or chalk. These nodules typically precipitate from groundwater in voids or as replacements for original organic material over millions of years.
Uses & applications
Used historically for stone tools (knapping) due to its conchoidal fracture. Currently used for lapidary work, decorative landscaping, and as a component in some industrial abrasives.
Geological facts
Chert is so hard it can scratch glass and steel. It was one of the most important materials for early humans to create arrowheads, scrapers, and blades because it breaks with razor-sharp edges.
Field identification & locations
Identify in the field by its waxy appearance on fresh surfaces, distinctive curved (conchoidal) fractures, and its ability to spark when struck with steel. Commonly found in gravel beds or weathered out of carbonate rock formations.
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