Obsidian slabs are volcanic glass.  They often have flow lines that create very interesting effects. For example, gold and silver sheen types display gold and silver chatoyance when polished.  Rainbow obsidian displays multiple colors.

Obsidian slabs can have a reputation for being difficult to polish.  With attention to a couple main details though, you can have a great time working with this material.

Coarse grits can create a fairly deep fracture zone that doesn’t come out when the stone is cut at finer grits.  Also, obsidian doesn’t polish well if it gets too hot.  If you don’t use super coarse grinding wheels, and keep your polishing wheel slow and wet, good results will easily be obtained!