Just as a note for those who didn't notice: the mafia specified osmium, which means that he would have assumed osmium was malleable and soft, which it obviously isn't being hard and brittle. I hardly think that he would have just said 'osmium'. It was specified to be a lynching. What else could he have done anyway? make an osmium sword? That would be too public for a mafia.

So, we can still know that the mafia was not knowledgable of the chemistry of osmium. Or that he was pretending not to be, which seems overly clever, and I don't think we have any chemists here who would obviously know osmium back to front anyway.