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 remaining stones appear to be coated in fine black oil.
Blackhaven, seat of House Dondarrion, has forbidding walls of black basalt.
The bridge at Breakwater Castle is made of black basalt.
In eastern Essos, Kayakayanaya has walls of black basalt, black iron, and yellow bone.