sedimentary
Chert (Jasper/Agate variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5 - 7 on the Mohs scale. Color: Opaque cream, grey, and tan with subtle banding. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Opaque cream, grey, and tan with subtle banding
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5 - 7 on the Mohs scale. Color: Opaque cream, grey, and tan with subtle banding. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary layers. This specimen appears water-worn, likely from a river or beach environment.
Uses & applications
Used historically for stone tools (arrowheads), as a minor gemstone when polished (tumbled stones), and in industrial abrasives or road construction aggregate.
Geological facts
Chert is one of the most durable sedimentary rocks. Because of its conchoidal fracture pattern, it was a primary material for prehistoric tool-making. High-silica rocks like this can spark when struck against steel.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, waxy feel. It is commonly found in gravel pits, riverbeds, and as nodules within limestone or chalk formations worldwide.
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