sedimentary
Chert (or Carnelian Agate)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown to orange-brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Reddish-brown to orange-brown
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown to orange-brown; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed primarily as a chemical sedimentary rock through the accumulation of silica-rich fluids or the replacement of limestone. Can also occur as nodules in volcanic rocks. This specimen shows iron oxide staining giving it the reddish hue.
Uses & applications
Used historically for stone tools (knapping) due to its sharp edges; used in jewelry as semi-precious gemstones (Carnelian), and as decor or landscaping stone.
Geological facts
The reddish color is caused by impurities of iron oxide (hematite). Ancient civilizations used this material to make seals, amulets, and arrowheads because it produces very sharp, glass-like edges when broken.
Field identification & locations
Identify by its waxy texture and conchoidal (shell-like) fracture patterns. It will scratch glass but cannot be scratched by a steel knife. Commonly found in riverbeds, glacial tills, or as nodules in limestone cliffs.
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