Quartzite (River Cobble)

Hardness: 7 (Mohs scale), Color: tan to light brown with darker grey patches, Luster: vitreous to dull, Crystal structure: granular/interlocking, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7

Quartzite forms when quartz-rich sandstone is subjected to intense heat and pressure (metamorphism). The specimen shown is a water-worn river cobble, suggesting it has been transported and smoothed by water over thousands of years.

Used as a high-durability construction aggregate, in the glass industry (if high purity), for railway ballast, and occasionally as decorative landscaping stone.

Quartzite is so hard that when it breaks, the fractures go through the quartz grains themselves rather than around the grain boundaries as they do in the parent sandstone. This rounded specimen is likely a 'glacial erratic' or river stone that has survived long-distance transport due to its extreme resistance to erosion.

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its grainy, crystalline texture. Found globally in areas of ancient mountain building; common in riverbeds and glacial deposits. For collectors, it is an example of a survivor rock.