Quartzite

Hardness: 7 on Mohs scale; Color: Typically white to gray, but can be pink, red, purple, or green due to impurities; Luster: Vitreous (glassy) to dull; Crystal structure: Crystalline, interlocking quartz grains; Cleavage: None (fractures conchoidally, though typically irregular due to granular nature); Specific gravity: 2.65

Quartzite forms from the metamorphism of sandstone, a quartz-rich sedimentary rock. Under intense heat and pressure, the quartz grains in sandstone recrystallize, fusing together to form a very dense, durable rock. The original pore spaces are eliminated, and the silica cement that held the sandstone grains together also recrystallizes, creating a strong, interlocking mosaic of quartz crystals. This process typically occurs during regional metamorphism associated with mountain-building events, or burial metamorphism. Geological age varies depending on the age of the protolith (sandstone) and the metamorphic event.

Used extensively in construction for dimension stone, flooring, wall cladding, and countertops due to its durability, hardness, and resistance to weathering. Because of its hardness, it is also used as an aggregate in road construction and railway ballast. Industrially, high-purity quartzite is a source of silica for glassmaking, ceramics, and metallurgical flux. Some colored and veined varieties are used as ornamental stones.

Quartzite is one of the most durable rocks found on Earth's surface, making it highly resistant to physical and chemical weathering. This is why it often forms resistant ridges and mountain peaks. Unlike marble, which is also a metamorphic rock used in construction, quartzite is very resistant to acidic weathering, making it more suitable for outdoor applications and kitchen countertops. Pure quartzite is composed of 90-99% quartz, making it chemically inert.

In the field, quartzite can be identified by its exceptional hardness (it will scratch glass and steel), its gritty, sugary texture (if fine-grained, can feel smooth), and its tendency to break across individual quartz grains rather than around them (unlike sandstone). It often exhibits a conchoidal fracture, though a granular texture can make this less obvious. It does not react with acid (unlike limestone or marble). Commonly found in mountain ranges and regions that have undergone significant tectonic activity, such as the Appalachian Mountains, Rocky Mountains, and various shield areas worldwide. For collectors, well-formed specimens with vibrant colors or interesting textures are prized. A good field test is to strike it with a hammer; it will often produce sparks due to the hardness of quartz.