Quartzite (River Cobble)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan to yellowish-brown due to iron staining; Luster: Dull to slightly waxy; Structure: Granitoid/Massive; Cleavage: None; SG: 2.6-2.7

Formation & Geological History

Formed from the metamorphism of quartz-rich sandstone under intense heat and pressure. This specific rounded shape indicates it was later eroded and smoothed by high-energy water action in a river or glacial environment.

Uses & Applications

Commonly used as crushed stone in construction, road ballast, or as decorative landscaping stones. Highly durable for outdoor use.

Geological Facts

Quartzite is so hard that it can scratch glass and steel. It is one of the most resistant rocks to chemical and physical weathering, which is why it often survives as rounded cobbles in stream beds.

Field Identification & Locations

Identify in the field by its extreme hardness and grainy, 'sugary' texture when freshly broken. Found globally in old mountain belts and river basins.

Identified on: 4/16/2026

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