Diamonds are pure carbon. Since humans are made up of carbon and we require carbon to live, it logically follows that diamonds should be perfectly safe to eat.
Seriously though, ingesting small amounts of rust (assuming it is iron oxide) won't hurt you, it takes much more than that to cause iron poisoning.
I wish I was rich enough to indulge in a diamond dinner, my plumber would freak out. And yes any health authority will tell you there is absolutely no problem with drinking rusty water from old pipes. You still wouldn't catch me dead using anything with rust on it in the kitchen. I'll go out of my way to replace something as simple as a plastic ruler into a stainless steel one, but I'm probably a minority.
I just think that the " there's worst out there, so what's the problem " is not a right way of thinking. To me it's like tossing a piece of garbage in the street because " 1 piece of garbage won't pollute the planet ". That's really my point, but I'm not on CC to preach to anyone. I just think the OP had a valid point. You could replace the word " pencil " with many more objects in different threads.
It's okay if some people don't agree with the OP, but it doesn't make it a frivolous post.
And as far as tossing trash goes... Don't let me catch you because if there is a way, I will pick it up, and crack you in the face with it... Or at least toss it in your car and give you a filthy look and tell you not to litter. That gets my blood boiling!