Diabase (also known as Dolerite)

Physical Properties

Hardness: 6-7 (Mohs scale); Color: Dark grey to greenish-black; Luster: Dull to sub-vitreous; Crystal Structure: Medium-grained with an ophitic texture (randomly oriented laths of plagioclase fieldspar); Specific Gravity: 2.7-3.3.

Formation & Geological History

Formed through the cooling of mafic magma in shallow intrusive bodies like dikes and sills. It is chemically similar to basalt but cooled more slowly, resulting in larger crystals. Most common in tectonic rift zones.

Uses & Applications

Commonly used as crushed stone for construction, road beds, and railroad ballast. Due to its durability, it is also used for gravestones and high-quality kitchen countertops (often sold as 'Black Granite').

Geological Facts

Stonehenge's inner circle 'bluestones' are a type of dolerite. It is one of the most common rocks found in the Palisades along the Hudson River.

Field Identification & Locations

Identify in the field by its high density, dark color, and 'salt and pepper' appearance of tiny interleaved crystals. It lacks the vesicles (gas bubbles) often found in basalt.

Identified on: 4/22/2026

Mode: Standard