igneous

Diorite (River Pebble)

Intermediate plutonic igneous rock

Hardness: 6-7 Mohs; Color: Salt-and-pepper (grey, black, white); Luster: Dull (weathered) to vitreous; Structure: Phaneritic (visible grains); Composition: Plagioclase feldspar, biotite, hornblende

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

Identified May 16, 2026More igneous →

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Physical properties

Hardness: 6-7 Mohs; Color: Salt-and-pepper (grey, black, white); Luster: Dull (weathered) to vitreous; Structure: Phaneritic (visible grains); Composition: Plagioclase feldspar, biotite, hornblende

Formation & geological history

Formed by the slow cooling of magma deep beneath the Earth's surface (intrusive). This specific specimen has been rounded and smoothed by water transport in a river or coastal environment.

Uses & applications

Commonly used as crushed stone for road construction, base material, or, in high-quality large slabs, as polished decorative stone for countertops known as 'black granite'.

Geological facts

Diorite was used 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' appearance with roughly equal parts light and dark minerals. Found in volcanic arcs and mountain building zones. Watch for the lack of quartz compared to granite.

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

Blue Apatite

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

mineral

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Magnetite

Magnetite (Fe3O4)

mineral