igneous
Granodiorite Pebble
Granodiorite
Hardness: 6-7 Mohs; Color: Salt and pepper (mottled grey, white, and black); Luster: Sub-vitreous to dull; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.6-2.7
- Hardness
- 6-7 Mohs
- Color
- Salt and pepper (mottled grey, white, and black)
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 Mohs; Color: Salt and pepper (mottled grey, white, and black); Luster: Sub-vitreous to dull; Structure: Phaneritic (coarse-grained); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the slow cooling of silica-rich magma deep within the Earth's crust (plutonic formation), typically in subduction zones above convergent plate boundaries. This specimen has been rounded by water erosion.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and building foundations; higher-quality slabs are used for countertops and architectural cladding.
Geological facts
Granodiorite is the intermediate rock between granite and diorite. The famous Rosetta Stone was carved from a granodiorite slab, not basalt as originally thought.
Field identification & locations
Identify in the field by its visible crystals of plagioclase feldspar (usually white), quartz (grey/glassy), and darker minerals like biotite or hornblende. Commonly found in riverbeds near mountainous igneous outcrops.
