sedimentary
Chert (variably called Jasper or Agate)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7.0 (Mohs). Color: Brown, tan, and dark gray bands. Luster: Waxy to vitreous. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Brown, tan, and dark gray bands
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7.0 (Mohs). Color: Brown, tan, and dark gray bands. Luster: Waxy to vitreous. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the accumulation of silica-rich marine organisms or chemical precipitation from hydrothermal fluids in rock cavities. This specimen appears water-worn, likely from a river or beach deposit.
Uses & applications
Historically used for stone tools (arrowheads); currently used in lapidary work, jewelry, and as tumbled decorative stones.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture creates extremely sharp edges; it is chemically identical to quartz but with a different internal structure.
Field identification & locations
Identify by its hardness (cannot be scratched by a steel nail) and its smooth, waxy surface. Commonly found in gravel beds, riverbanks, and limestone outcrops.
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