Quartzite

Hardness: 7 on Mohs scale. Color: Typically white to light grey, but can be shades of pink, yellow, or green due to impurities. Luster: Vitreous to greasy. Crystal structure: Typically granular, interlocking quartz grains. Cleavage: None (fractures conchoidally). Specific Gravity: 2.65.

Quartzite forms from sandstone that has been metamorphosed through heat and pressure, usually associated with regional metamorphism caused by tectonic plate compression. The intense conditions recrystallize the quartz grains and the former cementing material, resulting in a dense, extremely hard rock. Geological age varies depending on the original sandstone and metamorphic event, ranging from Precambrian to relatively recent.

Industry: Crushed for road construction, railway ballast, and as an abrasive. Also used as a silica source for glassmaking and ceramics. Construction: Dimension stone for flooring, wall cladding, countertops, and roofing due to its durability and aesthetic appeal. Jewelry/Decorative: Occasionally used as a gemstone or in decorative objects, particularly colorful varieties. Collecting: Valued by collectors for its hardness and sometimes vibrant colors.

Quartzite is one of the most durable and resistant rocks found on Earth. It is highly resistant to weathering, which is why it often forms resistant ridgelines and mountain peaks. The 'Q' in the QFL (Quartz, Feldspar, Lithics) diagram for sandstone classification highlights the dominance of quartz that leads to quartzite formation. Some famous examples of quartzite mountain ranges include parts of the Appalachian Mountains in the eastern United States.

To identify quartzite in the field, look for its extreme hardness (it will scratch glass and cannot be scratched by a knife). Observe its granular, interlocking texture, where individual sand grains are no longer visible as separate particles but are fused together. It often breaks across grain boundaries rather than around them, giving a smoother, conchoidal fracture. Common locations include metamorphic belts worldwide, areas of ancient mountain building, and regions where sandstone formations have undergone intense pressure and heat. It can be found in places like the Appalachian Mountains, parts of Scandinavia, and various locations across North America, Europe, and Asia. A field tip is to look for its glassy luster on fresh breaks and its resistance to weathering, often appearing as highly resistant outcrops.