Quartzite (River Cobble)

Physical Properties

Hardness: 7 on Mohs scale; Color: White to tan with iron staining; Luster: Vitreous to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: Indistinct/Conchoidal fracture; Specific gravity: 2.63 - 2.65

Formation & Geological History

Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains and silica cement to recrystallize into an interlocking mosaic of quartz crystals. These specific specimens are water-worn 'river rocks' deposited in the Quaternary period.

Uses & Applications

Used primarily in construction as crushed stone for road gravel and rail ballast. Smooth cobbles are popular in landscaping and decorative garden features. High-purity quartzite is used to produce ferrosilicon and industrial sand.

Geological Facts

Quartzite is so tough that it often outlasts the surrounding rocks during erosion. While sandstone breaks around its grains, quartzite is so well-fused that it breaks cleanly through the grains when struck.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by steel) and its crystalline, sugary appearance on fresh surfaces. Often found as rounded cobbles in riverbeds, glacial tills, or decorative mulch beds as seen here.

Identified on: 4/14/2026

Mode: Standard