Quartzite pebble

Hardness: 7 (Mohs scale); Color: Gray to bluish-gray with iron staining; Luster: Vitreous/waxy; Structure: Non-foliated granoblastic; Specific Gravity: 2.6-2.7.

Formed through the metamorphism of quartz sandstone under high heat and pressure, typically during mountain-building events (orogeny). This specific piece is a 'river rock' or 'alluvial pebble' that has been water-worn and smoothed in a high-energy environment like a stream or river bed.

Commonly used in landscaping, as railroad ballast, in construction as crushed stone, and sometimes as a source of high-purity silica for glassmaking.

Quartzite is so tough and abrasion-resistant that it often remains as pebbles and boulders long after other rocks have eroded away. It is harder than steel and will scratch glass easily.

Identify in the field by its extreme hardness (cannot be scratched by a knife) and its sugary or glassy appearance on a fresh break. It is ubiquitous in riverbeds worldwide, particularly in areas with ancient mountain ranges.