sedimentary
Chert (Jasper/Agate)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, and yellow with banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, and yellow with banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, and yellow with banding; Luster: Waxy to vitreous; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the accumulation of silica-rich micro-organisms (radiolarians or diatoms) on the seafloor or through the replacement of limestone by silica-rich fluids. Can be billions of years old.
Uses & applications
Historically used for making stone tools due to sharp edges; currently used for lapidary art, jewelry, and as tumbled decorative stones.
Geological facts
Chert and its varieties like Jasper and Agate are among the most durable materials on Earth. They are often found as 'river stones' because their hardness allows them to outlast softer surrounding rocks during erosion.
Field identification & locations
Identify in the field by its smooth, waxy surface and conchoidal (shell-like) fractures. It will easily scratch glass. Common in riverbeds, gravel pits, and sedimentary outcrops.
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