Quartzite (River Cobble)

Physical Properties

Hardness: 7 on Mohs scale; Color: White to light grey with tan/pink iron staining; Luster: Vitreous to dull/sugary; Crystal Structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Formation & Geological History

Formed through the metamorphism of quartz-rich sandstone. Under intense heat and pressure, individual quartz grains recrystallize and fuse together. This specific specimen has been further shaped and smoothed by water transport in a river or glacial environment.

Uses & Applications

Used as a decorative stone in landscaping, a durable construction aggregate, and occasionally in glassmaking or as a refractory material due to its high silica content.

Geological Facts

Quartzite is so hard and chemically resistant that it often forms the caps of ridges and mountains because it resists weathering better than the rocks around it. It is one of the most durable materials on Earth's surface.

Field Identification & Locations

Identify in the field by its extreme hardness (it will easily scratch glass and stainless steel knife blades) and its lack of reaction to acid. Look for a 'sugary' texture on fresh breaks where grains have fused together. Commonly found in riverbeds and glacial tills.

Identified on: 4/21/2026

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