metamorphic
River Cobbles (Quartzite and Sandstone)
Metamorphosed Quartz-rich Sedimentary Rock
Hardness: 7 (Mohs scale); Color: White, tan, reddish-brown, grey; Luster: Dull to waxy; Texture: Fine to medium-grained; Shape: Rounded to sub-rounded due to fluvial erosion.
- Hardness
- 7 (Mohs scale)
- Color
- White, tan, reddish-brown, grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: White, tan, reddish-brown, grey; Luster: Dull to waxy; Texture: Fine to medium-grained; Shape: Rounded to sub-rounded due to fluvial erosion.
Formation & geological history
These specimens are river-worn stones. The large white central stone is likely Quartzite, formed from the metamorphism of quartz sandstone under intense heat and pressure. Other stones include sandstone and siltstone, predominantly from Proterozoic or Paleozoic eras.
Uses & applications
Commonly used in landscaping, architectural filler (gabions), decorative garden paths, and as drainage aggregate in construction.
Geological facts
Quartzite is so hard that it can scratch glass and steel. The rounded shape of these stones indicates they have traveled many miles in a high-energy water environment, such as a riverbed or a glacial outwash plain.
Field identification & locations
Identify in the field by their rounded, smooth texture and extreme hardness. They are ubiquitous in riverbeds and glacial deposits throughout North America and Europe.
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