igneous
River Pebble (Grantoid/Quartzite)
Quartz-rich igneous rock (Granitoid)
Hardness: 6-7 Mohs; Color: Mottled grey, tan, and dark brown; Luster: Matte to waxy (water-worn); Structure: Phaneritic/Crystalline; Cleavage: None/Massive.
- Hardness
- 6-7 Mohs
- Color
- Mottled grey, tan, and dark brown
- Luster
- Matte to waxy (water-worn)
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Physical properties
Hardness: 6-7 Mohs; Color: Mottled grey, tan, and dark brown; Luster: Matte to waxy (water-worn); Structure: Phaneritic/Crystalline; Cleavage: None/Massive.
Formation & geological history
Formed from the slow cooling of silica-rich magma beneath the Earth's surface, later eroded and rounded by transport in a river or glacial environment. Aged likely between 100 million to 1 billion years old.
Uses & applications
Commonly used in landscaping, decorative ground cover, aquarium substrates, or as a tactile pocket stone.
Geological facts
River pebbles become smooth through a process called saltation, where they are bounced along riverbeds, grinding against other rocks to create a rounded, oval shape.
Field identification & locations
Identify by its rounded shape, lack of sharp edges, and mottled mineral grains. Found universally in active or ancient riverbeds and glacial deposits.
