is there a limitation to the amount of jewel i can collect?

Yes. Whenever you have 50 jewels, you need to convert them.
If you are not a donator, you also have a total limit.
When you have enough geos, troops or city value, you can't jewelhunt anymore until you lose a lot.

what's enough?

I'm not sure what the total limit is, but have a look here:
http://www.gewar.net/forums/showpost.php?p=213&postcount=3