Igneous

Diorite (Polished Cobble)

Diorite (Intermediate intrusive igneous rock)

Hardness: 6-7 on Mohs scale; Color: Mottled 'salt and pepper' appearance (black, dark green, and white/grey); Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (coarse-grained); Specific gravity: 2.8 - 3.0.

Hardness: 6-7 on Mohs scale
Luster: Sub-vitreous to dull

Identified March 29, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the slow cooling of silicate magma below the Earth's surface, typically in volcanic arcs or above subduction zones. This specimen appears water-worn or mechanically polished from an ancient riverbed or glacial deposit.

Uses & applications

Used primarily as crushed stone for road construction, as architectural 'black granite' for countertops and floor tiles, and occasionally for carving or lapidary work.

Geological facts

Diorite was extremely popular in Ancient Egypt and Mesopotamia for sculptures because of its extreme hardness; the famous Code of Hammurabi was inscribed on a diorite stele.

Field identification & locations

Identify by its 'salt and pepper' mixture of light plagioclase feldspar and dark amphibole or pyroxene minerals. Found in large mountain ranges such as the Andes, or as glacial erratics in the Northern US and Europe.

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Blue Apatite

Fluorapatite (Ca5(PO4)3F)

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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

Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

mineral