Quartzite

Hardness: 7 on Mohs scale; Color: typically white to gray, often with tan or brown iron staining; Luster: vitreous to dull; Crystal structure: trigonal (granular mosaic); Cleavage: none (fractures across grains); Specific gravity: 2.6-2.7

Formed through the regional metamorphism of quartz-rich sandstone. High heat and pressure cause individual quartz grains to recrystallize and fuse into a dense, interlocking network. Often found in orogenic (mountain-building) belts dating from the Precambrian to Cenozoic eras.

Extensively used as decorative stone in architecture and landscaping, as crushed stone for road construction and railway ballast, and in the manufacturing of glass and ceramics due to high silica content.

Quartzite is so tough and resistant to chemical weathering that it often forms the resistant ridges or peaks of mountain ranges. Unlike sandstone, which breaks around grains, quartzite breaks directly through the quartz grains because the bonds between grains are as strong as the grains themselves.

Identify in the field by its extreme hardness (will easily scratch glass) and its granular, sugary texture that lacks the visible pores of sandstone. It is commonly found in mountainous regions such as the Appalachians in the US, the Alps in Europe, and throughout the Canadian Shield. Collectors look for smooth, river-worn cobbles or crystalline fractures.