Well, putting jewellery on your teeth is really not a big deal as far as "safety" goes. Fillings, bridges, crowns and what-not are tried and tested.
Only thing is, do you really think you'll want jewellery on your teeth for the rest of your life? Just do it, as long as you're aware of the fact that you probably will want to get rid of it some day... That's when the problems start.
What a strange power there is in clothing. Isaac Bashevis Singer
I'm not sure if we're talking about the same thing but my friend wanted to get a "diamond" put on her tooth. It really wasn't a diamond per se though, just a colored stone. It wasn't an actual procedure either, I'm pretty sure it's just put on with a glue that will disolve naturally in a month or something like that.
I expect a false jewel could be glued on--braces for straightening teeth are glued on. However, it would probably feel pretty weird, and might look like you had food stuck in your teeth. Also, if it wasn't glued down securely, or was hard to clean around adequately, it could promote decay.
I am thinking this sounds like a bad idea. I don't think it would look very attractive (too "bling-y") and any way you might attempt it could damage your teeth.