sedimentary

Rounded quartz-rich sandstone or quartzite cobble

Quartz-dominated siliciclastic rock; likely sandstone, possibly weakly metamorphosed quartzite

AI-generated identification · may be incorrect

Tan-gray, fine-grained, dull to subtly vitreous surface with rounded edges from stream transport; predominantly quartz with cementing silica, iron oxides, and minor feldspar/clay. Estimated hardness 6–7 Mohs; massive texture, no obvious cleavage, and moderate-to-high density.

Identified August 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Most likely deposited as sand in an ancient river, beach, or delta, then compacted and cemented; if quartzite, later heat and pressure recrystallized it. Its precise geological age and setting cannot be determined from the photograph.

Uses & applications

Common sandstone is used for building stone, paving, aggregate, and landscaping; quartzite is used for durable countertops, flooring, road aggregate, and decorative stone. This unremarkable cobble has little commercial or jewelry value.

Geological facts

Quartz-rich rocks commonly develop resistant rounded cobbles because quartz withstands chemical and mechanical weathering. A fresh broken surface and acid test can help distinguish sandstone from carbonate rocks.

Field identification & locations

Look for visible sand grains and gritty texture in sandstone; quartzite is more uniformly crystalline and breaks across grains rather than around them. Common worldwide in streambeds and sedimentary basins; estimated value is $1–$10 as a specimen, usually only its decorative or educational interest.