Quartzite

Hardness 7 (Mohs); color: white, gray, and tan; luster: vitreous to dull; crystal structure: trigonal (recrystallized); cleavage: indistinct; specific gravity: 2.6-2.7.

Formed through the metamorphism of quartz sandstone under intense heat and pressure, causing the original quartz grains to recrystallize into a interlocking mosaic of quartz crystals. Typically associated with convergent plate boundaries.

Used in construction as road ballast, roofing granules, and decorative stone. Sometimes used for countertops or as a source of silica for glass manufacturing.

Quartzite is so tough that it often cracks through the quartz grains rather than along their boundaries, unlike the sandstone it originated from. Some of the oldest mountain ranges are composed of quartzite due to its extreme resistance to erosion.

Identify by its extreme hardness (cannot be scratched by steel) and lack of reaction to acid. It often appears sugary or grainy. Found in mountainous or glaciated regions worldwide.