sedimentary
Chert (Common Opal/Jasper variety)
Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Yellow-brown, tan, and translucent grey; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Yellow-brown, tan, and translucent grey
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Yellow-brown, tan, and translucent grey; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich fluids in limestone or volcanic ash layers. Over millions of years, the silica precipitates and hardens into microcrystalline quartz. This specimen shows characteristics of common opal or jasper-chert blends found in high-silica volcanic environments.
Uses & applications
Used historically for stone tools and arrowheads due to its sharp edges when broken. Today it is used in lapidary work for cabochons, landscaping, and as a collector's specimen.
Geological facts
Chert is so hard it can scratch glass and steel. In prehistoric times, it was a primary material for making survival tools because its conchoidal fracture allows for the creation of extremely sharp, razor-like edges.
Field identification & locations
Identify in the field by its waxy appearance, smooth 'shell-like' fracture surfaces (conchoidal), and extreme hardness. Often found in riverbeds or desert pavements in volcanic regions.
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