sedimentary
Chert (Yellow Jasper)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Tan, yellow, or mustard brown. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Tan, yellow, or mustard brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Tan, yellow, or mustard brown. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed by the precipitation of silica from groundwater in sedimentary environments or as biochemical deposits on the ocean floor. Most specimens found in surface gravels date from various Phanerozoic periods.
Uses & applications
Historically used for making stone tools (arrowheads). Modern uses include landscaping, as a lapidary material for tumbling, and in jewelry as yellow jasper.
Geological facts
Chert is so hard it can create sparks when struck against steel. It was one of the most important materials for early human technology due to its sharp conchoidal edges.
Field identification & locations
Identified by its smooth, waxy surface and curved, glass-like fracture patterns. Commonly found in riverbeds, glacial tills, and as nodules within limestone or chalk.
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