Yes, baby proof. But, I think that after 6 months, a crawling baby is the perfect time to start developing limits - I hate the idea of rules so young. But just think of them as limits, not rules or mandates. They seem to have an easier time if there are limits from the beginning - not many, but a few.
The fireplace seems to draw babies - and it just isn't safe. So, after I did this first idea for about a week - I never had a problem. My baby would crawl on by after I did this...
The first idea - make a loud sound. Right as they are about to touch the fireplace - a "honk" or a duck-sounding, loudish "aaa!" A loud - but short sound. I know that kids aren't animals - but the simple Pavlovian method of doing something unpleasant helps them associate surprising sound with touching the fireplace. They are so little, that they learn pretty quickly. Does that make sense?
The second idea - 20 seconds of holding their arms still. So, right after they have gotten to the fireplace you (instead of saying no - or saying anything) just go to their side and looking away, you hold their arms still at their side. After they realize they are stuck, count to 20 (it will make them a bit frustrated) then calmly say, if you touch the fireplace, you can't use your arms. Short. Sweet. And again they associate something they would rather not with an unpleasant moment. This idea comes from The New First Three Years.
I would only do one at a time and see which one your baby responds to. Good luck!
If anyone has another idea, please share!
Painting: The Delphic Sibyl by Michelangelo detail from the Sistine Chapel