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

Identified March 20, 2026More igneous →

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Physical properties

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.

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

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Magnetite

Magnetite (Fe3O4)

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