sedimentary
Chert (Jasper variety) with Iron Oxide staining
Cryptocrystalline Quartz (SiO2) with Goethite/Limonite impurities
Hardness: 6.5–7 on Mohs scale. Color: Brown, tan, and grey with possible translucent zones. Luster: Waxy to dull. Crystal structure: Cryptocrystalline micro-textures. Cleavage: None (conchoidal fracture). Specific gravity: 2.6–2.7.
- Hardness
- 6
- Color
- Brown, tan, and grey with possible translucent zones
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale. Color: Brown, tan, and grey with possible translucent zones. Luster: Waxy to dull. Crystal structure: Cryptocrystalline micro-textures. Cleavage: None (conchoidal fracture). Specific gravity: 2.6–2.7.
Formation & geological history
Formed through the accumulation of silica-rich fluids in sedimentary environments or the replacement of limestone. This specimen appears water-worn, suggesting it was found in a river or coastal deposit of Quaternary to Holocene age, though the parent rock could be hundreds of millions of years old.
Uses & applications
Used historically for stone tools due to its conchoidal fracture. In modern times, it is used as a semi-precious gemstone in lapidary work, tumbling, and as decorative aggregate in landscaping.
Geological facts
Chert is so durable that it often survives as 'river stones' long after the surrounding limestone or shale has eroded away. The distinctive indentation or 'hole' in this specimen is likely a natural weathering feature where a softer mineral inclusion or fossil fragment dissolved out.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, waxy surface when wet. Common in riverbeds and gravel pits globally. Collectors look for unique shapes, 'pockets', or interesting color banding.
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