igneous
Granite Cobble
Phaneritic Plutonic Felsic Rock
Hardness: 6-7 Mohs; Color: Pinkish-orange (K-feldspar), grey/translucent (quartz), and black/dark brown (biotite/amphibole); Luster: Dull/Waxy (weathered) to Vitreous (fresh surfaces); Grain: Medium to coarse phaneritic crystals; Specific Gravity: 2.63 - 2.75.
- Hardness
- 6-7 Mohs
- Luster
- Dull/Waxy (weathered) to Vitreous (fresh surfaces)
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Physical properties
Hardness: 6-7 Mohs; Color: Pinkish-orange (K-feldspar), grey/translucent (quartz), and black/dark brown (biotite/amphibole); Luster: Dull/Waxy (weathered) to Vitreous (fresh surfaces); Grain: Medium to coarse phaneritic crystals; Specific Gravity: 2.63 - 2.75.
Formation & geological history
Formed through the slow cooling and crystallization of silica-rich magma deep within the Earth's crust (plutonism). This specimen has been rounded through fluvial or glacial erosion, as evidenced by its smooth, water-worn shape. Age can range from hundreds of millions to billions of years depending on the regional craton.
Uses & applications
Used widely in construction (crushed stone, aggregate, dimension stone), kitchen countertops when quarried in slabs, and as decorative landscaping stones. Smooth river-worn cobbles are often used in masonry and landscaping.
Geological facts
Granite makes up much of the continental crust. Because it is high in quartz and feldspar, it is exceptionally resistant to weathering compared to other rocks, which is why it often survives as rounded cobbles long after softer rocks have eroded away.
Field identification & locations
Identify in the field by its 'salt and pepper' appearance (mixed light and dark crystals) and its inability to be scratched by a steel knife. It is found globally, especially in mountain ranges, shield areas like Canada/Africa, and stream beds or glacial till. Collectors look for unique mineral inclusions or rare colors.
