sedimentary
Quartz Pebble (Chert/Chalcedony Gravel)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: yellowish-tan with dark brownish/gray mottling; Luster: earthy to dull waxy; Crystal structure: trigonal (microcrystalline/cryptocrystalline); Cleavage: none (conchoidal fracture when broken); Specific gravity: ~2.65
- Hardness
- 6
- Color
- yellowish-tan with dark brownish/gray mottling
- Luster
- earthy to dull waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: yellowish-tan with dark brownish/gray mottling; Luster: earthy to dull waxy; Crystal structure: trigonal (microcrystalline/cryptocrystalline); Cleavage: none (conchoidal fracture when broken); Specific gravity: ~2.65
Formation & geological history
Formed via precipitation of silica in sedimentary environments or as rounded pebbles weathered out from quartz-rich bedrocks, streams, or glaciated gravel deposits. Over time, physical weathering rounded the specimen into a pebble.
Uses & applications
Commonly used as construction aggregate, landscaping gravel, driveway gravel, and as road base material. Well-rounded quartz pebbles are also used in lapidary art or tumbling.
Geological facts
Stream-rolled quartz pebbles like this are amongst the most common rocks on Earth's surface. The yellowish staining is typically due to iron oxides (limonite/goethite) leaching into the porous outer surface.
Field identification & locations
Identified by its hardness (scratches glass), yellowish-tan weathered exterior, rounded river-worn texture, and dense microcrystalline quartz structure. Commonly found in riverbeds, glacial till, beaches, and gravel pits worldwide.
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