igneous
Granodiorite with Quartz Veining
Granodiorite (containing Plagioclase Feldspar, Quartz, and Biotite/Hornblende) with Quartz (SiO2) veins
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (mottled grey, white, and black) with white/translucent veins. Luster: Dull to vitreous. Crystal structure: Phaneritic (medium to coarse-grained) igneous texture. Cleavage: Indistinct in hand specimen, but plagioclase shows two directions. Specific gravity: 2.65-2.75.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Salt-and-pepper (mottled grey, white, and black) with white/translucent veins. Luster: Dull to vitreous. Crystal structure: Phaneritic (medium to coarse-grained) igneous texture. Cleavage: Indistinct in hand specimen, but plagioclase shows two directions. Specific gravity: 2.65-2.75.
Formation & geological history
Formed through the slow cooling of silica-rich magma deep beneath the Earth's surface (plutonic). The white veins formed later when hydrothermal fluids filled fractures in the solidifying rock. Common in continental crust and mountain ranges from various geological eras, including the Mesozoic and Paleozoic.
Uses & applications
Used primarily in heavy construction, road base (crushed stone), and as dimension stone (granite-style countertops or monuments). Often too coarse for jewelry unless unique polished cabochons are made.
Geological facts
Granodiorite is the most common plutonic rock in many of the world's mountain belts. The famous Rosetta Stone was carved from a similar dark granodiorite rock, though it was historically misidentified as basalt.
Field identification & locations
Identify in the field by its speckled 'salt-and-pepper' appearance and high quartz content; quartz veins will scratch steel. Commonly found in glaciated areas (as erratics), riverbeds, and large batholiths like the Sierra Nevada or Scottish Highlands.
