igneous

Granodiorite

Granodiorite (phaneritic intrusive igneous rock)

Hardness: 6-7 Mohs; Color: Salt-and-pepper (white-grey with black flecks); Luster: Dull to vitreous; Structure: Phaneritic (coarse-grained crystals); Cleavage: None (rock breaks irregularly, though component minerals have cleavage); Specific Gravity: 2.6-2.7.

Hardness: 6-7 Mohs
Color: Salt-and-pepper (white-grey with black flecks)
Luster: Dull to vitreous

Identified March 21, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of silica-rich magma deep underground (plutonic). Found in large batholiths often associated with mountain-building events (orogenic belts). These can range from Precambrian to Cenozoic in age.

Uses & applications

Used extensively in construction as crushed stone for road base, concrete, and railroad ballast. When polished, it is sold as 'Granite' for countertops, floor tiles, and grave markers.

Geological facts

The famous Plymouth Rock is a glacial erratic made of granodiorite. It is chemically intermediate between granite and diorite, containing more plagioclase feldspar than orthoclase.

Field identification & locations

Identify by its 'salt and pepper' look where white/grey grains (feldspar/quartz) are mixed with dark specks (biotite/hornblende). Common in the Sierra Nevada mountains and Pacific Northwest beaches as water-worn cobbles. Collectors look for large, well-defined mineral grains.

Other igneous specimens

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

mineral

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

Magnetite

Iron(II,III) oxide (Fe3O4)

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

mineral

Magnetite

Magnetite (Fe3O4)

mineral