igneous

Granodiorite (River Pebble)

Granodiorite

Hardness: 6-7 (Mohs); Color: Mottled grey, white, and black; Luster: Dull (weathered) to vitreous; Structure: Phaneritic (medium-grained crystalline); Cleavage: None (fractures irregularly). Heavy and dense.

Hardness: 6-7 (Mohs)
Color: Mottled grey, white, and black
Luster: Dull (weathered) to vitreous

Identified May 21, 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. This specific specimen has been further shaped by fluvial (river) erosion, which smoothed its edges over hundreds to thousands of years.

Uses & applications

Used primarily as crushed stone for road construction, building materials, and as decorative river rock in landscaping.

Geological facts

Granodiorite is the main component of many famous mountain ranges and was the material used to carve the Rosetta Stone. It has more plagioclase feldspar than orthoclase, distinguishing it from true granite.

Field identification & locations

Look for a salt-and-pepper appearance with interlocking crystals and a rounded, smooth surface if found near water. It is very common in glacial till and riverbeds across North America.

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