Quartzite

Hardness: 7 (Mohs), Color: Grey/Greenish-grey with white streaks, Luster: Vitreous to dull, Structure: Granular/crystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone under high heat and pressure, typically during mountain-building events (orogeny). Earth's quartzite deposits range from billions of years old to relatively young formations.

Used as crushed stone for road construction, railroad ballast, and building facades. In its purer forms, it is used for countertops, ornamental stone, and as a source of silica in glass manufacturing.

Quartzite is so hard that it can scratch steel and glass easily. Due to its resistance to chemical weathering, quartzite often forms the caps of ridges and mountains where softer rocks have eroded away.

Identify in the field by its extreme hardness and sugary, crystalline appearance on fresh surfaces. It is found globally in mountainous regions such as the Appalachians, Alps, and Himalayas. Collectors look for pieces with unique mineral inclusions or banding.