Oceanic crust at mid-ocean ridges is primarily formed by basalt, a dark-colored, fine-grained igneous rock.

Here's a breakdown:

* Color: Basalt is typically dark gray to black in color.

* Type: Basalt is an extrusive igneous rock, meaning it formed from the cooling and solidification of lava on the Earth's surface.

At mid-ocean ridges, magma rises from the Earth's mantle and erupts along the spreading center. This lava cools and solidifies to form new oceanic crust.