igneous
River Cobble (Quartzite / Porphyritic Igneous Rock)
Siliceous rock / Water-worn lithic fragment
Hardness: 6-7 on the Mohs scale. Color: Brownish-green with mottled lighter patches. Luster: Vitreous to dull when wet/dry. Crystal structure: Cryptocrystalline to microcrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: ~2.6-2.7.
- Hardness
- 6-7 on the Mohs scale
- Color
- Brownish-green with mottled lighter patches
- Luster
- Vitreous to dull when wet/dry
Identified More igneous →
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Physical properties
Hardness: 6-7 on the Mohs scale. Color: Brownish-green with mottled lighter patches. Luster: Vitreous to dull when wet/dry. Crystal structure: Cryptocrystalline to microcrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: ~2.6-2.7.
Formation & geological history
Formed through regional or contact metamorphism of sandstone into quartzite, or through the cooling of silica-rich magma, subsequently weathered, eroded, and smoothed by water transport in a river environment over thousands to millions of years.
Uses & applications
Used primarily in landscaping, decorative stone, and as natural river gravel. Educational and general rock-collecting value.
Geological facts
River cobbles like this are shaped by hydraulic action and abrasion as they are tumbled against other rocks in fast-moving water, which rounds their sharp edges into smooth, pebble-like shapes.
Field identification & locations
Commonly found in stream beds, riverbanks, and glacial outwash plains. Easily identified by its smooth, water-worn texture and resistance to scratching due to high quartz content.
