metamorphic
Rounded quartzite cobble
Quartzite, predominantly microcrystalline SiO₂
AI-generated identification · may be incorrectTypically Mohs hardness 7, very tough, with a sugary or smooth texture and white, gray, tan, or iron-stained yellow coloration; vitreous to dull luster and no distinct cleavage. The rounded shape indicates water or glacial abrasion, while dark patches may be weathering or accessory minerals.
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Physical properties
Typically Mohs hardness 7, very tough, with a sugary or smooth texture and white, gray, tan, or iron-stained yellow coloration; vitreous to dull luster and no distinct cleavage. The rounded shape indicates water or glacial abrasion, while dark patches may be weathering or accessory minerals.
Formation & geological history
Forms when quartz-rich sandstone is recrystallized under metamorphic heat and pressure, commonly during mountain-building; its exact geological age cannot be determined from the photograph. The specimen was likely transported into a stream, beach, or glacial deposit.
Uses & applications
Used as durable aggregate, riprap, paving stone, and occasionally decorative stone; attractive pieces are collected but are generally not gem-quality.
Geological facts
Quartzite commonly rings when struck and resists scratching by steel, unlike sandstone, which is softer and sheds grains. Iron oxides account for the yellow-brown staining.
Field identification & locations
Common in metamorphic terrains and derived river or glacial gravels worldwide, including the Appalachians, Rocky Mountains, Alps, and Scandinavian regions. Confirm by testing: it should not fizz in dilute hydrochloric acid and should scratch glass; a streak or broken surface should look granular and quartz-rich.
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