Basalt is a stone that is found on Volcanoes. When mined with a pickaxe, it drops one block of Basalt Cobblestone, which can be smelted back into Basalt, then crafted into Basalt Bricks, in a similar manner to Stone Brick. It cannot be used to make tools or stairs. Currently the only use for Basalt is blast resistance and decoration.
For information on the blast resistance, see the Basalt Brick.