Basalt is a hard igneous rock formed directly from lava. It generally has a blackish or dark gray color, and can be used as structural or surface masonry. It is similar to gabbro, except that basalt is homogeneous in appearance and does not have visible grains. Basalt is formed when lava cools and hardens at the Earth's surface, whereas gabbro is formed underground.

Both masonry and surface cladding can use basalt. The material designation does not require the stone to carry the building loads.

Its generally fine texture contrasts with the visible crystalline grains of gabbro. Color can be similar, so darkness alone is insufficient to distinguish them.