Quartzite

Physical Properties

Hardness: 7 on Mohs scale; Color: typically white to gray, often with yellow or brown staining from iron oxides; Luster: vitreous to dull; Crystal structure: hexagonal (microcrystalline); Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7.

Formation & Geological History

Formed through the regional metamorphism of quartz sandstone. Intense heat and pressure cause the individual quartz grains to recrystallize and fuse together into a dense, hard rock. This process can occur over various geological ages, often in mountain-building zones.

Uses & Applications

Used primarily in construction for road ballast, railroad track ballast, and as a decorative stone in landscaping. High-purity quartzite is used as a source of silica for glassmaking and industrial ceramics.

Geological Facts

Quartzite is so tough that it often forms the caprock or ridges in mountain ranges because it resists weathering much better than surrounding rocks like shale or limestone. When struck with a hammer, it typically breaks through the quartz grains rather than around them, distinguishing it from sandstone.

Field Identification & Locations

Identify it in the field by its extreme hardness (it will easily scratch glass) and its granular, sugary texture. It is commonly found in metamorphic terranes worldwide, such as the Appalachian Mountains or the Alps. For collectors, look for pieces with interesting mineral inclusions or distinct crystalline structures.

Identified on: 4/13/2026

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