Granodiorite (Riverbed Cobble)

Physical Properties

Hardness: 6-7 Mohs; Color: Salt and pepper (gray, white, black) with occasional pink spots; Luster: Phaneritic (crystalline) and dull when weathered; Crystal structure: Phaneritic (medium-grained); Cleavage: Distinct in feldspar grains; Specific gravity: Approx 2.6-2.7

Formation & Geological History

Formed from the slow cooling of silica-rich magma beneath the Earth's surface in continental crust or volcanic arcs. This specific specimen shows signs of fluvial erosion (rounding by water). Geological age varies by source location but is typically Precambrian to Tertiary.

Uses & Applications

Used widely in construction as crushed stone, road base, and ornamental architectural stone (termed 'white granite' in trade). Small smooth cobbles are used in landscaping and aquariums.

Geological Facts

Granodiorite is the most common intrusive rock in the Earth's crust. The famous Rosetta Stone was carved from a slab of granodiorite, not basalt as famously thought.

Field Identification & Locations

Identify by its 'salt and pepper' appearance and visible interlocking crystals. Look for small flecks of pink (orthoclase feldspar) amidst mostly gray/white (plagioclase). Common in mountain ranges and riverbeds downstream from batholiths.

Identified on: 4/22/2026

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