sedimentary
Chert (Jasper/Agate variety)
Microcrystalline quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Variable (opaque tan, brown, and reddish due to iron oxides); Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65
- Hardness
- 6
- Color
- Variable (opaque tan, brown, and reddish due to iron oxides)
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Variable (opaque tan, brown, and reddish due to iron oxides); Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65
Formation & geological history
Formed through the accumulation of silica-rich fluids in sedimentary environments or by the replacement of calcium carbonate in limestone. The rounded shape suggests secondary weathering in a river or coastal environment. These rocks can range from millions to billions of years old.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges. Modern uses include tumbling for decorative gravel, landscaping, jewelry (as semi-precious gemstones), and industrial abrasives.
Geological facts
Chert was one of the first materials used by early humans for tools like spearheads and scrapers. Because it is harder than steel, it was also used in 'flintlock' firearms to create sparks to ignite gunpowder.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. It is commonly found in gravel pits, riverbeds, and as nodules within limestone or chalk formations. Collectors often look for colorful banding or translucency.
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