Quartzite

Hardness: 7 (Mohs scale), Color: Light gray, white, or tan with pinkish hues, Luster: Vitreous to dull, Crystal Structure: Granoblastic, Cleavage: None (conchoidal fracture)

Formed when quartz-rich sandstone is subjected to intense heat and pressure (metamorphism). The individual quartz grains recrystallize into a dense, interlocking mosaic. Formed typically during mountain-building events over millions of years.

Used extensively in construction as crushed stone for road grit and railway ballast, as a decorative building stone for countertops and walls, and historically utilized for tool-making.

Quartzite is so hard that it will scratch glass and even high-carbon steel. Unlike sandstone, which breaks around grains, quartzite breaks through the individual quartz grains due to its high density.

Identified in the field by its sugary texture and extreme hardness. Look for it in riverbeds or mountainous regions with exposed metamorphic rock. Collectors value cobbles for lapidary work if they show unique colors or translucent properties.