Quartzite

Hardness: 7 (Mohs scale), Color: White to light gray, Luster: Vitreous to dull, Crystal structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7.

Formed from the regional or contact metamorphism of quartz-rich sandstone. Under intense heat and pressure, original quartz sand grains recrystalize and fuse together into a dense, hard interlocking network of crystals.

Used extensively in road construction and railway ballast due to its hardness and durability. High-purity quartzite is used to produce glass, ferrosilicon, and silicon metal. It is also popular as a decorative stone in architecture and countertops.

Quartzite is so tough that it often survives intense weathering better than surrounding rocks, frequently forming the tops of ridges and mountains. It will scratch glass easily, unlike limestone or marble which it can sometimes resemble.

Identify in the field by its extreme hardness and tendency to break through quartz grains rather than around them (unlike sandstone). It is found globally in mountainous regions and areas of ancient metamorphic activity.