Yellow Quartzite (River Cobble)

Physical Properties

Hardness: 7 (Mohs scale); Color: Yellow/Tan/Orange (due to iron oxidation); Luster: Vitreous to waxy; Crystal structure: Hexagonal/trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & Geological History

Formed through the metamorphism of quartz sandstone under high heat and pressure, causing sand grains to recrystallize into a dense network. This specific specimen has been further shaped and polished by fluvial (river) erosion, likely over hundreds to thousands of years.

Uses & Applications

Used primarily in construction as crushed stone, railroad ballast, and decorative landscaping. High-purity quartzite is used to produce silicon for electronics and glass.

Geological Facts

Quartzite is so hard that it can scratch steel and glass easily. It is often tougher than the parent sandstone because the individual grains are fused together rather than just chemically cemented.

Field Identification & Locations

Identify by its extreme hardness, gritty look but smooth feel, and the way it breaks through the quartz grains rather than around them. Common in riverbeds and glacial deposits across North America.

Identified on: 4/24/2026

Mode: Standard