Here you can find several obsidian rocks meshes and PBR materials.

Triangle counts:

Obsidian rock 1 - 2054

Obsidian rock 2 - 8770

Obsidian rock 3 - 7970

Obsidian rock 4 - 6470

Obsidian rock 5 - 2620

Obsidian rock 6 - 2000

Obsidian rock flat 1 - 740

Obsidian rock flat 2 - 2010

Obsidian rock small 1 - 480

Obsidian rock small 2 - 170

Obsidian rock small 3 - 110

Obsidian rock small 4 - 760

Obsidian rock small 5 - 1700

Obsidian rock small 6 - 980

Obsidian rock small 7 - 1180

Obsidian rock small 8 - 830