Quartzite

Hardness: 7 on the Mohs scale (due to interlocking quartz grains); Color: Varies widely, often white, gray, pink, or red, but can be green, blue, or yellow due to impurities; Luster: Vitreous (glassy) to dull; Crystal structure: Crystalline, interlocking granular texture; Cleavage: None, generally breaks across grains; Specific Gravity: 2.65

Quartzite forms from the metamorphism of pure quartz sandstone. Under intense heat and pressure, the quartz grains in the sandstone recrystallize, fusing together and interlocking, creating a rock that is much harder and more durable than the original sandstone. The geological age of quartzite varies greatly depending on the age of the protolith sandstone and the metamorphic event it underwent, ranging from Precambrian to Cenozoic.

Quartzite is highly valued in construction for its durability, hardness, and aesthetic appeal. It's used for flooring, countertops, wall cladding, and roofing tiles. Crushed quartzite is used as railroad ballast, in road construction, and as a raw material for glass manufacturing (if pure enough). In industry, it can be used in abrasive applications. Due to its resistance to weathering and attractive appearance, it is sometimes used for decorative landscaping and as dimension stone.

Pure quartzite is nearly 100% quartz. The intense metamorphism that forms quartzite can obliterate the original sedimentary structures like bedding planes and ripple marks present in the sandstone. When quartzite breaks, it typically fractures across the quartz grains rather than around them, which distinguishes it from sandstone.

To identify quartzite in the field, look for its granular, sugary texture, often appearing glassy. It's very hard, so it will scratch glass and cannot be scratched by a steel knife. Unlike sandstone, when broken, it generally fractures through the grains rather than around them, creating a conchoidal or irregular fracture. Common locations include regions that have undergone regional metamorphism, often associated with mountain-building events. It's found in many continents, including the Appalachians (USA), parts of Europe, and Brazil.