igneous

Granodiorite (Water-worn Pebble)

Granodiorite

Hardness: 6-7 on Mohs scale. Color: Mottled grey, white, and black. Luster: Dull to phaneritic (crystalline). Crystal structure: Phaneritic (medium-grained intrusive). Cleavage: Varies by mineral (feldspar vs quartz). Specific gravity: 2.6-2.7.

Hardness: 6-7 on Mohs scale
Color: Mottled grey, white, and black
Luster: Dull to phaneritic (crystalline)

Identified April 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the slow cooling of silica-rich magma beneath the Earth's surface (plutonic). This specific specimen appears water-worn, indicating erosion and transport by a river or coastline over thousands of years.

Uses & applications

Used in heavy construction, road base, as crushed stone, and occasionally for salt-and-pepper style countertops or decorative landscaping stone.

Geological facts

Granodiorite is the major component of the famous 'Stone Mountain' in Georgia and was the stone used for the Rosetta Stone. It is intermediate in composition between granite and diorite.

Field identification & locations

Identify by its 'salt and pepper' appearance and visible interlocking crystals of white/grey plagioclase feldspar and dark biotite or hornblende. Commonly found in mountain ranges and riverbeds.

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