Basalt is a volcanic rock found in the known world.
The curtain wall of Moat Cailin was made of enormous blocks of black basalt. When wet, the massive stones which remain appear to be coated in fine black oil.
Blackhaven, seat of House Dondarrion, has forbidding walls of black basalt, and Kayakayanaya has walls of black basalt, black iron, and yellow bone.
The bridge at Breakwater Castle is made of black basalt.