Quartzite

Hardness: 7 (Mohs scale), Color: light grey to brownish-grey, Luster: dull to vitreous, Crystal structure: Trigonal (granular texture), Cleavage: none, Specific gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause quartz grains to recrystallize and fuse together into a dense, interlocking structure. This specific specimen appears to be water-worn, indicating it was smoothed by river or coastal erosion.

Used in construction as crushed stone for road gravel and railway ballast. Due to its durability, it is also used for decorative building stone, countertops, and flooring.

Quartzite is so hard and durable that it often remains as resistant ridges or outcrops when softer surrounding rocks erode away. Unlike sandstone, when quartzite breaks, it fractures through the sand grains rather than around them.

Identify in the field by its superior hardness (cannot be scratched by steel) and lack of reaction to acid. It is commonly found in older mountain belts or as rounded pebbles in streambeds. Collectors often find these as 'river rocks'.