Quartzite

Hardness: 7 on the Mohs scale; Color: White to light gray; Luster: Vitreous to dull; Crystal structure: Non-foliated, interlocking quartz grains; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formed through the regional or contact metamorphism of quartz-rich sandstone. High heat and pressure cause the original quartz grains to recrystallize and fuse together. Can range from Precambrian to younger geological ages.

Extensively used in construction as crushed stone for road bases, railway ballast, and roofing granules. High-purity quartzite is used to produce silica sand for glassmaking and silicon metal. It is also used as a decorative stone in landscaping and architecture.

Quartzite is so tough that it often survives intense weathering better than other rocks, frequently forming the caprock on mountain ridges or resistant outcrops. It is often confused with marble, but unlike marble, it will not fizz when exposed to acid and cannot be scratched by a steel knife.

Identify in the field by its extreme hardness (scratches glass) and its tendency to break through the sand grains rather than around them (unlike sandstone). Found globally in mountainous regions and areas with ancient continental shields. Collectors look for pieces with unique colors from mineral impurities.