Quartzite

Hardness: 7 on Mohs scale. Color: Typically white or gray, but can be shades of pink, yellow, or red due to impurities. Luster: Vitreous (glassy) to greasy. Crystal Structure: Non-foliated, granular crystalline structure with interlocking quartz grains. Cleavage: None. Specific Gravity: 2.65.

Quartzite forms from the metamorphism of sandstone, which is a sedimentary rock composed primarily of quartz sand. Under intense heat and pressure, the quartz grains in the sandstone recrystallize, fusing together to form a much harder, more durable rock. The process typically occurs during regional metamorphism associated with mountain building events, or contact metamorphism near igneous intrusions. Geological age varies depending on the original sandstone formation and the metamorphic event.

Quartzite is valued in construction for its durability, hardness, and resistance to weathering. It is used for flooring, countertops, wall cladding, and roofing tiles. Crushed quartzite is used as a road aggregate and railway ballast. Historically, some indigenous cultures used quartzite to make tools, similar to flint.

Quartzite is one of the hardest and most durable rocks on Earth. Its extreme resistance to erosion often leads to it forming prominent ridges and peaks in mountainous regions. The transformation from sandstone to quartzite is so complete that the original sedimentary structures are usually destroyed, making it difficult to distinguish individual sand grains.

In the field, quartzite can be identified by its exceptional hardness (it will scratch glass and cannot be scratched by a knife) and its distinct granular, sugary texture, which is due to the interlocking quartz crystals. Unlike sandstone, quartzite will not feel gritty when rubbed and will break across the grains, rather than around them. It typically has a glassy luster when freshly broken. Common locations include regions that have undergone significant tectonic activity, such as mountain ranges (e.g., Appalachians, Rockies) and shields. For collectors, well-formed specimens showcasing distinct color variations or structural features are sought after. Look for fresh breaks to best assess its texture and luster.