Quartzite

Hardness: 7 on Mohs scale; Color: typically white to gray, often with tan or yellowish staining from iron oxides; Luster: Vitreous to dull; Crystal structure: Trigonal (microcrystalline/granoblastic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

Formed through the regional or contact metamorphism of quartz-rich sandstone. Under intense heat and pressure, original quartz grains recrystallize and fuse together into a dense, interlocking mosaic of crystals. Most commonly associated with mountain-building events (orogenies).

Widely used as crushed stone for road construction and railway ballast. Due to its extreme hardness and resistance to chemical weathering, it is used for floor tiles, countertops, and as decorative stone in landscaping. High-purity quartzite is used in the manufacture of glass and silicon.

Quartzite is so tough that it often outlasts the surrounding rocks in a landscape, forming prominent ridges and peaks. It is often confused with marble, but unlike marble, it will not react with acid and can easily scratch glass.

Identify in the field by its grainy, sandpaper-like surface that nonetheless breaks through the grains rather than around them (unlike sandstone). Look for its characteristic hardness and lack of reaction to dilute hydrochloric acid. It is found in ancient mountain ranges worldwide, including the Appalachians, the Alps, and the Scottish Highlands.