Quartzite Cobble

Hardness: 7 (Mohs scale); Color: light grey to tan; Luster: dull to vitreous; Crystal structure: Granular/Crystalline; Specific gravity: approx. 2.6

Formed when quartz-rich sandstone is subjected to high heat and pressure, causing the original quartz grains to recrystallize into a dense, interlocking mosaic of quartz crystals. This specific specimen has been rounded by fluvial (river) or coastal erosion processes.

Used in construction as road ballast, gravel, and railroad bedding. When crushed and high in purity, it is used in the glass industry and to produce silicon. Smaller cobbles are popular in landscaping and as decorative garden stones.

Quartzite is one of the most durable rocks on Earth's surface. It is often harder than the steel blade of a pocket knife. The rounding of this stone indicates it has spent significant time being tumbled in water.

Identified by its extreme hardness (cannot be scratched by steel), granular 'sugary' texture on fresh breaks, and lack of reaction to acid. Common in riverbeds, glacial deposits, and mountain ranges.