hhhmmm,
my off the cuff response would have been bludgening to bludgening/peicing/slashing, but that aint correct by RAW.
As for lighting oil shale, DM's call. See if google or someone can give you an ignition point then make your decision about what can trigger it. my (again off the cuff) thoughts that you wouldn't be able to light it via anything short of magic that specifically start things burning or a very large leat souce (which gives you other problems as well).