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