Quartzite

Hardness: 7 on Mohs scale. Color: Often white or light gray, but can be shades of pink, yellow, red, or green due to impurities. Luster: Vitreous (glassy) to greasy. Crystal structure: Typically massive and granular, with interlocking quartz grains. Cleavage: None, fractures conchoidally. Specific gravity: 2.65.

Formed from sandstone that has been metamorphosed through intense heat and pressure, usually associated with regional metamorphism in mountain-building events (orogenic belts) or contact metamorphism near igneous intrusions. The original quartz grains in the sandstone recrystallize and interlock, forming a very strong, dense rock. Its geological age varies widely depending on the age of the original sandstone and the metamorphic event.

Used extensively as an architectural stone for flooring, wall cladding, countertops, and decorative features due to its durability and aesthetic appeal. Also used as a crushed stone for road construction, railway ballast, and in concrete aggregates. High-purity quartzite is used in the glassmaking industry as a source of silica, and in metallurgy as a flux.

Quartzite is one of the most durable and resistant rocks found on Earth due to its interlocking quartz grain structure, making it highly resistant to weathering and erosion. It is often mistaken for marble, but can be distinguished by its much greater hardness (marble is much softer and will scratch easily). Unlike sandstones that can be friable, quartzite is exceptionally tough.

To identify in the field, look for a very hard, granular rock that will scratch glass and steel. It often has a splintery or conchoidal fracture. Its interlocking texture is a key indicator. It does not react to acid like marble or limestone. Commonly found in ancient mountain ranges, such as the Appalachian Mountains in North America, and in areas of uplifted and eroded continental crust worldwide. Collectors often seek out particularly colorful or aesthetically pleasing specimens for display due to its natural beauty and durability.