smash it to get a couple of fairly flat surfaces (one on each side ofthe break) then rub the two halfs against each other to make them flat. you could also just rub it round and round in circles on an old concrete paving slab, maybe with some water added to make the job a bit quicker and easier, to make it flat (you have the added bonus that way of ending up with a beautifully polished concrete slab, honestly, it comes up a treat). i've used all sorts of random lumps of rock as sharpening stones over the years, pretty much anything works to an extent, it's just that some work better than others. i'd say get a knife that you're not bothered about (there's loads in the kitchen drawer, just don't let the mother/girlfriend/wife find out) and have a play with whatever stones you can find
stuart