sedimentary
Chert (Common Opal/Jasper variant)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs); Color: Tan, beige, grey, and waxy white; Luster: Waxy to vitreous; Structure: Cryptocrystalline/microcrystalline; Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Tan, beige, grey, and waxy white
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs); Color: Tan, beige, grey, and waxy white; Luster: Waxy to vitreous; Structure: Cryptocrystalline/microcrystalline; Cleavage: None (conchoidal fracture).
Formation & geological history
Forms through the accumulation of silica-rich fluids in limestone/chalk or through the burial and compaction of silica-rich marine organisms (diatoms/radiolarians) over millions of years.
Uses & applications
Historically used for stone tools and arrowheads; currently used in jewelry (as jasper/opalite), landscaping, and as an industrial abrasive.
Geological facts
Chert is so hard it can create sparks when struck against steel. Flint is a common dark-colored variety of chert. This specific specimen appears to have a high silica content giving it a waxy, semi-translucent appearance often called common opal.
Field identification & locations
Look for a smooth, waxy texture and shell-like (conchoidal) curved breaks. Found globally in sedimentary basins. Collectors should check for sharp edges and translucency when held to light.
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