Seal your dorfs in and set the whole fuggin world on fire. It looks ridiculously cool!
... Or make magma smelters/forges, if you haven't already. Being able to smelt ores by looking angrily at them is pretty rad.
And finally, if you're feeling really dorfy, use it in a mold with water to cast a large obsidian object, which you preferrably drop on someone from high above. It'll even leave a nice stamp on the ground, due to the obsidian being counted as natural walls. Just don't forget that it'll hammer right through any floors with impunity, since i've forgotten it multiple times myself.
...
Oh wait, you said least destructive. But don't tell me you aren't tempted.