Quartzite

Hardness: 7 (Mohs), Color: White, gray, pink, red, black, or yellowish, often with natural iron staining. Luster: Vitreous (glassy) to greasy. Crystal structure: Typically massive, interlocking grains of quartz. Cleavage: None. Specific Gravity: 2.65.

Quartzite forms from sandstone that has been metamorphosed through heat and pressure, typically associated with regional metamorphism caused by tectonic plate collisions. The original quartz grains in the sandstone recrystallize, forming a much harder and more durable rock with interlocking quartz crystals. Its geological age varies greatly depending on the original sandstone and the metamorphic event.

Quartzite is used extensively in construction for countertops, flooring, wall cladding, and roofing because of its hardness, durability, and aesthetic appeal. It is also used as a crushed stone for rail tracks and road beds. In specific applications, high-purity quartzite is used in the production of silica brick, ferrosilicon, and silicon metal.

Pure quartzite is generally white or light-colored, but iron oxides can impart pink or reddish hues. Other impurities can lead to yellow, green, blue, or even black colors. Its extreme hardness makes it resistant to weathering and erosion, leading to the formation of prominent ridges and mountains in many geological settings. Unlike sandstones, quartzite fractures across the original grains due to the complete recrystallization.

In the field, quartzite can be identified by its exceptional hardness (it will scratch glass and steel easily), its crystalline, sugary texture (like cemented sugar grains), and the way it fractures directly through the grains, rather than around them as sandstone would. Often, it has a glistening appearance due to the freshly exposed quartz crystals. It is commonly found in mountain ranges and areas that have undergone significant tectonic activity, such as the Appalachian Mountains in the eastern United States, various ranges in Scotland, and parts of India and Brazil.