sedimentary
Chert (Yellow Jasper/Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 on the Mohs scale. Color: Tan, yellow-orange, or brownish-yellow due to iron oxide inclusions. Luster: Dull to waxy. Crystal Structure: Trigonal (microscopic). Cleavage: None; exhibits prominent conchoidal (shell-like) fracture. Specific Gravity: 2.6–2.7.
- Hardness
- 6
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5–7 on the Mohs scale. Color: Tan, yellow-orange, or brownish-yellow due to iron oxide inclusions. Luster: Dull to waxy. Crystal Structure: Trigonal (microscopic). Cleavage: None; exhibits prominent conchoidal (shell-like) fracture. Specific Gravity: 2.6–2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) on the ocean floor, or via the chemical replacement of limestone/chalk by silica-rich groundwater. These specimens can range from the Paleozoic to the Cenozoic era.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when broken. Today, it is used as an abrasive, in construction aggregate, and lower-grade specimens like this are used for landscaping or rock tumbling. High-quality versions are used in lapidary for jewelry.
Geological facts
Because of its extreme hardness and conchoidal fracture, chert was one of the most important materials for early humans to manufacture tools and weapons. Yellow variations are often colloquially called 'Jasper' if they are opaque.
Field identification & locations
Identify in the field by its smooth, waxy surface and curved, glass-like fracture patterns when chipped. It will easily scratch a knife blade or glass. Commonly found in stream beds, gravel pits, or within limestone outcrops. Collectors look for pieces with unique banding or patterns.
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