Mahogany Obsidian Slabs from Oregon. Obsidian is volcanic glass. It often has flow lines and transparent areas that create interesting effects.
Obsidian has a reputation for being difficult to polish, usually because too coarse a grit can create fairly deep fracture zones that won’t come out when the stone is cut at finer grits. Obsidian also doesn’t polish well if it gets too hot. So if you don’t use super-coarse grinding wheels and you keep your polishing wheel slow and wet, great results will easily be obtained.