Quartzite

Hardness: 7 (Mohs scale), Color: Pinkish-gray to brown, Luster: Vitreous to dull, Crystal structure: Granular/Crystalline, Specific Gravity: 2.6-2.7

Formed from the metamorphism of quartz sandstone under high heat and pressure, typically during mountain-building events. Geological age varies by location (frequently Precambrian to Paleozoic).

Used as a decorative stone in construction, road ballast, industrial glass manufacturing (silica source), and as a durable material for countertops.

Quartzite is so tough it often breaks through the individual sand grains rather than around them, creating a fracture surface that looks like glass in high-quality specimens.

Identify by its extreme hardness (cannot be scratched by a steel blade) and lack of fizzing with acid. Common in mountainous regions like the Appalachians or the Scottish Highlands.