igneous

Diabase

Diabase (also known as Dolerite)

Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-black with light flecks; Luster: Sub-vitreous to dull; Crystal structure: Phaneritic (medium-grained) with ophitic texture; Specific gravity: 2.7-3.3; Cleavage: Two directions at nearly 90 degrees (plagioclase/augite).

Hardness: 6-7 (Mohs scale)
Color: Dark grey to greenish-black with light flecks
Luster: Sub-vitreous to dull

Identified April 12, 2026More igneous →

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Physical properties

Formation & geological history

Formed by the relatively rapid cooling of basaltic magma within shallow intrusions like sills and dikes. It is intermediate in grain size between basalt and gabbro. Most prominent specimens date from the Mesozoic era, specifically associated with the breakup of Pangaea.

Uses & applications

Extensively used as 'trap rock' for crushed stone in road construction, railroad ballast, and concrete aggregate. High-quality slabs are sometimes used as building veneer or durable headstones.

Geological facts

Diabase is what makes up the famous Palisades Sill along the Hudson River in New York/New Jersey. It is also the primary material used for the inner 'blue stones' at Stonehenge in England.

Field identification & locations

Identify in the field by its dense, 'salt and pepper' appearance and its extreme toughness when hit with a rock hammer. It is commonly found in rift basins worldwide, such as the Triassic basins of the US East Coast.

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