igneous

Granodiorite

Granodiorite (Na,Ca)(Si,Al)4O8

Hardness: 6-7 (Mohs scale), Color: Salt and pepper (mottled grey, black, and white), Luster: Vitreous to dull, Crystal Structure: Phaneritic (coarse-grained), Cleavage: Perfect in feldspars, none in quartz, Specific Gravity: 2.6-2.7

Identified April 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the slow cooling of magma deep underground (intrusive). It is typical of the subduction zone magmatism and the roots of volcanic arcs. Found in massive batholiths that can be hundreds of millions of years old.

Uses & applications

Commonly used as crushed stone for road construction, building materials, paving stones, and sometimes polished as 'granite' for countertops and monuments.

Geological facts

Granodiorite is the rock that makes up the famous Rosetta Stone, which was essential for deciphering Egyptian hieroglyphs. It is intermediate in composition between granite and diorite.

Field identification & locations

Identify by its 'salt and pepper' appearance with more plagioclase feldspar than orthoclase. It is commonly found in the Sierra Nevada mountains of California and the Scottish Highlands. Look for visible crystals of quartz, feldspar, and dark minerals like biotite or hornblende.

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