Voidstone

Voidstone is a decorative block added by Quark. It will spawn in any End biome. It is immune to the Ender Dragon. Voidstone can be crafted into Stairs, Slabs, Vertical Slabs and Walls. It can also be polished, with its stair, slab, vertical slab variants. Polished Voidstone can be made into Bricks, with stairs, slabs, vertical slabs and walls. Also available are pavement, chiseled bricks, pillar and speleothem forms.

Before 1.16 it was called Basalt and would spawn in clusters in The Nether.

Recipe
Quark's recipes to uncraft slabs and stairs are only available for 1.9 to 1.12.