igneous

Diabase (Trap Rock)

Diabase (also known as Dolerite)

Hardness: 6-7 on Mohs scale; Color: Dark gray, green-black to black; Luster: Dull to sub-vitreous; Crystal structure: Fine to medium-grained (interlocking plagioclase and pyroxene); Specific gravity: 2.7-3.3.

Hardness: 6-7 on Mohs scale
Color: Dark gray, green-black to black
Luster: Dull to sub-vitreous

Identified March 22, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the shallow cooling of mafic magma in dikes and sills. These are intrusive rocks that cool faster than gabbro but slower than basalt. Often associated with volcanic activity and continental rifting during the Mesozoic Era.

Uses & applications

Extensively used as crushed stone for construction, road base (railroad ballast), and as ornamental stone for building facades or memorials. It is extremely durable and weather-resistant.

Geological facts

The famous Palisades Sill along the Hudson River is primarily composed of diabase. Some Stonehenge stones (called 'bluestones') are actually varieties of diabase/dolerite transported from Wales.

Field identification & locations

Identify by its 'salt and pepper' or dark greenish appearance with a lack of visible quartz. In the field, look for characteristic columnar jointing or blocky fracturing. Common in the Triassic-Jurassic basins of the Eastern US and Europe.

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

Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

mineral

Blue Apatite

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

mineral

Magnetite

Magnetite (Fe3O4)

mineral