sedimentary
Chert Pebble (Desert Varnish / Iron-Stained)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale. Color: Dark brown to black weathering rind/varnish with yellowish-brown to orange iron-stained underlying siliceous matrix. Luster: Dull to waxy or sub-vitreous. Cleavage: None (conchoidal fracture). Specific Gravity: ~2.65.
- Hardness
- 7 on Mohs scale
- Luster
- Dull to waxy or sub-vitreous
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Physical properties
Hardness: 7 on Mohs scale. Color: Dark brown to black weathering rind/varnish with yellowish-brown to orange iron-stained underlying siliceous matrix. Luster: Dull to waxy or sub-vitreous. Cleavage: None (conchoidal fracture). Specific Gravity: ~2.65.
Formation & geological history
Formed via the accumulation of microscopic silica-rich radiolarian or diatomaceous remains, or inorganic precipitation of silica in sedimentary environments. The dark exterior developed through surface weathering and deposition of iron and manganese oxides (desert varnish or stream-tumbled rind).
Uses & applications
Historically used for stone tools due to conchoidal fracturing. Modern uses include landscaping gravel, construction aggregate, and lapidary tumbling for hobbyists.
Geological facts
Chert pebbles are highly resistant to weathering, allowing them to travel great distances in river systems while retaining their hardness and rounded, smooth pebble shape.
Field identification & locations
Identified by its high hardness (scratches glass readily), smooth river-worn texture, conchoidal fracture pattern if broken, and dark iron oxide surface staining. Extremely common in riverbeds, gravel pits, beaches, and desert pavements worldwide.
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