Basalt

Basalt is a type of stone found deep underground in the Overworld. It takes slightly longer to break than regular stone. It can also be found in the Nether, albeit less extreme in generation.

Breaking
Basalt can be mined with any kind of pickaxe

Natural generation
Basalt starts generating in the Overworld at around Y=20 and starts to replace the majority of the stone generated below Y=14.

In the nether, Basalt generates about the same as in the Overworld, but in lower quantities.