Quartzite

Hardness: 7 on Mohs scale. Color: Typically white to gray, but can be shades of pink, yellow, or red due to impurities. Luster: Vitreous to greasy. Crystal structure: Granular, interlocking quartz grains. Cleavage: None. Specific gravity: 2.65.

Quartzite forms from the metamorphism of sandstone, a sedimentary rock. This process occurs under intense heat and pressure, often associated with tectonic plate collisions. During metamorphism, the individual quartz grains recrystallize, forming a dense, interlocking mosaic of quartz crystals. The original sedimentary structures, such as bedding planes, are usually obliterated. Geological age varies depending on the original sandstone and the metamorphic event, ranging from Precambrian to relatively recent.

Quartzite is extremely durable and weather-resistant, making it excellent for construction materials like paving stones, roofing tiles, and architectural facing. It is also used in the manufacturing of silica bricks for high-temperature furnaces, as an abrasive, and in some forms, as a decorative stone for countertops and flooring. High-purity quartzite is used in the electronics industry and for making glass.

Quartzite is one of the hardest and most resistant rocks found on Earth's surface. Its durability often means it forms prominent ridges and peaks in mountainous regions. The transformation from sandstone to quartzite is so complete that, unlike sandstone, when quartzite breaks, it fractures through the quartz grains, not around them.

To identify quartzite in the field, look for its granular, sugary texture and its extreme hardness – it will scratch glass and cannot be scratched by a steel knife. It often appears as massive, homogeneous rock bodies. Common locations include regions that have undergone significant mountain-building events, such as the Appalachian Mountains in North America, parts of the Scottish Highlands, and various shields worldwide. For collectors, well-formed specimens can be cut and polished, sometimes revealing interesting patterns or colors, especially if impurities are present.