Quartzite

Hardness: 7 (Mohs scale), Color: White to light grey with tan iron-staining, Luster: Vitreous to dull, Crystal Structure: Hexagonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Quartzite forms from the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the sand grains to recrystallize and fuse together, forming a dense, interlocking matrix. It is found in mountain belts and shield areas worldwide.

Used as a crushed stone for road construction, railroad ballast, and as a decorative building stone. Highly pure varieties are used to produce silica sand for glassmaking and industrial silicon.

Quartzite is so tough that it often survives as resistant ridges or hilltops while surrounding rocks erode away. It is often confused with marble, but can be distinguished by its inability to be scratched by a steel blade.

Identify by its high hardness (it will scratch glass) and lack of reaction to acid. Look for a granular, sugary texture where grains are no longer distinct but fused. It is extremely common in the Appalachian Mountains and the Lake Superior region.