Diabase

Diabase, also called dolerite, is a dark, medium-grained intrusive igneous rock with the same chemical composition as basalt and gabbro. It crystallizes at shallow depths, giving it a grain size between fine basalt and coarse gabbro.

It is dominated by calcium-rich plagioclase feldspar and pyroxene, with a characteristic interlocking 'ophitic' texture in which feldspar laths are enclosed by pyroxene. The rock is dense, dark grey to greenish-black, and extremely tough.

In North America the term diabase is standard, while in Britain and much of the world the same rock is called dolerite.

Diabase forms when basaltic magma intrudes into cracks and bedding planes in the shallow crust, creating dikes and sills that cool faster than deep plutons but slower than surface lava flows. This intermediate cooling rate produces its characteristic medium grain size and ophitic texture.

It is widespread in continental rift zones and large igneous provinces. Famous examples include the Palisades Sill along the Hudson River, the dikes of the Mesozoic rifting in eastern North America, and the dolerite sills of the Karoo in South Africa and Tasmania.

Look for a hard, heavy, dark grey to greenish-black rock with a medium grain size in which small, blocky crystals are just visible to the naked eye or with a hand lens. A magnifier may reveal the ophitic texture of feldspar laths in pyroxene.

It is dense and rings when struck. Look-alikes include basalt (finer-grained, often vesicular) and gabbro (coarser-grained). Diabase sits between the two in texture.

Weathered diabase may show greenish alteration and a rounded, spheroidal weathering pattern.

Diabase is one of the most important crushed-stone (trap rock) materials, valued for its hardness and durability as railroad ballast, road aggregate, and concrete stone. Its toughness makes it excellent for high-wear surfaces.

Polished diabase and dolerite are used as black dimension stone for countertops, monuments, and gravestones, sometimes marketed as 'black granite.' Historically, dolerite was used for prehistoric tools and some of the bluestones at Stonehenge are dolerite. It has no gemstone use.