Capntastic, please refrain from insulting and badgering other members. I'd like to say something along the lines of 'particularly when dealing with a subject you're not familiar with', but it's really just a general rule and I'd be something of a hypocrite for saying that.
To clear up some doubt, syndromes are not currently capable of transferring via solid objects. Which is to say, a log of poison wouldn't be able to do anything, but a puddle or a mist of poison would be more than capable of inflicting its effects. If you create a special rock that makes every living creature that comes in contact with it rot, then anybody could pick it up and haul it around without anything happening to them. You can even have bolts made out of the stuff get stuck in you without any adverse effects (well, aside from having a bolt sticking out of you).
However, if you were in the position to boil that same rock into a vapor state or melt it into a fluid state, that very same material would be able to infect people and the rotting would commence.
Because of this, weapons made out of a poisonous material do not do anything special, and do not infect others. However, if you have a standard-material weapon with some gooey poison smeared on it, then you can attack people and inject the venom (see
this thread for more details).
Also, since syndromes are transmitted via contaminants and since it's possible to create syndromes that transmit through contact with the substance, it
is possible to create an invulnerable creature that is specifically affected by that syndrome. I suppose it could also be possible to create some creature that is 'natural' in most aspects, but has vital bodyparts made from said indestructible material. This would make it possible for a creature to be damaged to some extent through standard methods, but not outright destroyed
As Capntastic said however, encasement in obsidian will almost certainly still be an instant death. Lava-related death is completely avoidable however. Heck, it's even done in the native game.