Spiderweb Obsidian Slabs
Obsidian is volcanic glass. It often has flow lines that create interesting effects. Gold sheen and silver sheen obsidians display gold and silver chatoyance when polished, rainbow obsidian displays different colors, etc. Obsidian has a reputation for being difficult to polish. Usually the reason for difficult polishing is that coarse grits can create a fairly deep fracture zone that doesn't come out when the stone is cut at finer grits. Another problem is obsidian doesn't polish well if it gets too hot. So, if you don't use super coarse grinding wheels and if you keep your polishing wheel slow and wet, good results will easily be obtained. I like cerium oxide for polishing obsidian.