sedimentary
Chert (Yellow Jasper/Common Opal variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Honey yellow to orange-brown; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 6
- Color
- Honey yellow to orange-brown
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 Mohs; Color: Honey yellow to orange-brown; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed from the accumulation of silica-rich microfossils or precipitation from silica-rich fluids in sedimentary basins or volcanic cavities. Often occurs as nodules or beds within limestone or volcanic rocks.
Uses & applications
Commonly used for lapidary work, jewelry (cabochons), and historically for stone tools due to its sharp conchoidal fracture.
Geological facts
Yellow jasper and chert were often used by indigenous peoples for arrowheads and scrapers. The coloration usually comes from trace inclusions of goethite or iron oxides.
Field identification & locations
Identified in the field by its waxy luster, smoothness, and the way it breaks into sharp, curved flakes (conchoidal fracture). It cannot be scratched by a steel knife. Common in river beds and gravel pits.
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