Diabase (Dolerite) Cobble

Physical Properties

Hardness: 6-7 on Mohs scale; Color: Dark grey to greenish-black with light flecks; Luster: Dull to sub-vitreous; Crystal structure: Fine to medium-grained ophitic texture; Specific Gravity: 2.7-3.3; Cleavage: Generally absent in the rock mass but present in component minerals.

Formation & Geological History

Formed through the cooling of mafic magma in shallow intrusive bodies like sills and dikes. This specific specimen has been further shaped into a smooth cobble by water erosion (fluvial or glacial action).

Uses & Applications

Commonly used as crushed stone for road construction, railroad ballast, and high-strength concrete. Fine specimens are sometimes used as ornamental 'river rocks' for landscaping.

Geological Facts

Diabase is chemically identical to basalt but cooled more slowly underground, resulting in larger crystals. It is the primary rock type found in the famous Palisades along the Hudson River.

Field Identification & Locations

Identify in the field by its dark color, high density, and the presence of light-colored plagioclase crystals intergrown with dark pyroxene (ophitic texture). Common in rift valleys and volcanic provinces.

Identified on: 4/26/2026

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