metamorphic
Tumbled gray quartzite or chert pebble
Quartzite (microcrystalline SiO₂; possibly chert)
AI-generated identification · may be incorrectHardness about 6.5–7 Mohs; gray to brown, matte-to-waxy luster, fine-grained and nonmetallic, with no visible cleavage. The rounded shape indicates natural water wear or tumbling; apparent patches may be quartz-rich bands or weathering.
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Physical properties
Hardness about 6.5–7 Mohs; gray to brown, matte-to-waxy luster, fine-grained and nonmetallic, with no visible cleavage. The rounded shape indicates natural water wear or tumbling; apparent patches may be quartz-rich bands or weathering.
Formation & geological history
Quartzite forms when quartz-rich sandstone is metamorphosed under heat and pressure; chert forms by silica precipitation or replacement in sedimentary settings. Exact age and identity cannot be determined from the photograph.
Uses & applications
Commonly used as aggregate, paving stone, and decorative rock; attractive pieces may be polished for lapidary work, though this small specimen has limited commercial value.
Geological facts
Quartz-rich rocks commonly scratch glass and resist weathering. A vinegar test is generally uninformative; a streak test and hardness test are more useful.
Field identification & locations
Common in stream gravels and glacial deposits worldwide. Confirm with a glass-scratch test, close inspection for sugary quartz grains, and acid testing only if carbonate material is suspected.
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