Quartzite

Hardness: 7 (Mohs), Color: Varies widely (white, gray, pink, yellow, brown, green, blue) often light, Luster: Vitreous to greasy, Crystal structure: Granular, interlocking quartz grains, Cleavage: None (fractures conchoidally), Specific gravity: 2.65

Quartzite forms from sandstone that has been metamorphosed through heat and pressure, typically associated with regional metamorphism during mountain-building events. It involves the recrystallization of quartz grains and siliceous cement, creating a much harder and more durable rock. Its geological age is as varied as the sedimentary sandstones from which it forms, ranging from Precambrian to Cenozoic.

Due to its hardness and durability, quartzite is widely used in construction as building stone, paving material, and in landscaping. Crushed quartzite is used as a road aggregate. High-purity quartzite is used in the manufacturing of ferro-silicon, silicon carbide, and as a flux in some metallurgical processes. Decorative varieties are used as countertops and wall cladding.

Quartzite is one of the most durable rocks on Earth, highly resistant to weathering and erosion. Its resistance to chemical weathering makes it a common ridge-former in many landscapes. Some varieties like 'Blue Quartzite' or 'Green Quartzite' are highly prized for ornamental use.

In the field, quartzite can be identified by its exceptional hardness (it will scratch glass and steel), its granular texture formed by interlocking quartz grains, and its tendency to fracture across the grains rather than around them like sandstone. It often has a vitreous luster. Common locations include regions with ancient mountain belts that have undergone significant metamorphism, such as the Appalachian Mountains in North America, parts of the Rocky Mountains, and various regions in Europe (e.g., Scotland, Scandinavia) and India. Collectors are often interested in samples displaying vibrant colors or unique textures.