sedimentary
Chert / Chalcedony (Agatized)
Silicon dioxide (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, and gray swirled; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Brown, tan, and gray swirled
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, and gray swirled; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the precipitation of silica from groundwater or oceanic fluids, often infilling cavities in limestone or replacing organic material. Typically dated back to various geological eras from Paleozoic to Mesozoic.
Uses & applications
Historically used for stone tools (flint); currently used for lapidary work, tumbling, and as decorative gravel.
Geological facts
This specimen appears to be a water-worn nodule. Chert is extremely durable and chemical resistant, which is why it commonly survives as river gravel long after other rocks erode.
Field identification & locations
Identify by its waxy texture, conchoidal (shell-like) fracture patterns, and hardness (it will scratch glass). Commonly found in riverbeds, glacial till, and sedimentary outcrops.
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