metamorphic

Weathered quartzite cobble

Quartzite, composed predominantly of recrystallized quartz (SiO₂)

AI-generated identification · may be incorrect

Typically Mohs hardness 7, nonmetallic vitreous to dull luster, granular interlocking quartz texture, and no obvious cleavage; this specimen is tan-brown from iron-oxide staining and surface weathering.

Identified September 16, 2026More metamorphic →

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Physical properties

Formation & geological history

Formed when quartz-rich sandstone was subjected to metamorphic heat and pressure, welding quartz grains into a durable rock; its precise age cannot be determined from the photograph. The rounded shape indicates transport by water, glacial ice, or other abrasion.

Uses & applications

Used as crushed aggregate, road stone, landscaping material, building stone, and occasionally as a decorative or lapidary material. Ordinary cobbles generally have little commercial value.

Geological facts

Quartzite commonly resists weathering better than surrounding rocks, producing rounded durable cobbles and prominent ridges. It can resemble hard sandstone, but quartzite typically breaks through grains rather than around them.

Field identification & locations

Likely identifiable by its extreme hardness, dense feel, quartz-rich granular appearance, and inability to scratch with a steel knife; a streak test is white or colorless. Common in metamorphic belts and derived river or glacial gravels worldwide.