Here's a little dentistry hack I did.

First off, I don't recommend anyone try doing your own dentistry based on my experience.

I had a dark, tea-stained porcelain cap on one of my front teeth. It was really starting to look bad, so I did some online searches hoping to learn how to bleach it to match my other teeth. I was disappointed to learn that you can't. I finally decided that what my cap needed was to be polished, so I inserted a pencil point-side-down into a cordless drill and spun the eraser to polish it (being careful not to rub against my natural teeth or gums). It took about 30 seconds and completely removed the discoloration.