metamorphic
Yellow Quartzite River Cobble
Meta-quartzite (SiO2 with iron oxide staining)
Hardness: 7 (Mohs scale); Color: Tan to yellowish-brown with orange iron staining; Luster: Dull to waxy when dry, vitrious when wet; Crystal structure: Non-foliated, interlocking microcrystalline quartz grains; Cleavage: None (conchoidal to irregular fracture)
- Hardness
- 7 (Mohs scale)
- Color
- Tan to yellowish-brown with orange iron staining
- Luster
- Dull to waxy when dry, vitrious when wet
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan to yellowish-brown with orange iron staining; Luster: Dull to waxy when dry, vitrious when wet; Crystal structure: Non-foliated, interlocking microcrystalline quartz grains; Cleavage: None (conchoidal to irregular fracture)
Formation & geological history
Formed from the regional or contact metamorphism of quartz-rich sandstone. This specimen is a 'river rock' or cobble, meaning it was eroded from its parent source and rounded over thousands of years through fluvial (water) transport. Many such stones date back to the Paleozoic or Mesozoic eras.
Uses & applications
Commonly used in landscaping, gravel for driveways, and as high-silica raw material for glass manufacturing and construction aggregates. Tumbled specimens are used in decorative crafts.
Geological facts
Quartzite is so durable that it often survives multiple cycles of erosion and redeposition. It is harder than steel and will easily scratch a glass plate. The yellow/orange color is a 'desert varnish' or rust-like staining caused by the oxidation of trace iron minerals.
Field identification & locations
Identify in the field by its inability to be scratched by a steel knife and its lack of visible grains (unlike sandstone). Found globally in riverbeds, glacial tills, and mountainous outcrops. It feels denser and smoother than most sedimentary rocks.
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