I know for visitor to Brasil you can only bring up to $500 in gifts without having to pay duties. What do I need to do if I want to bring in an engagement ring for my girlfriend it's about 10K, do I need to declare this and have to pay duties on it?

I think you answered your own question.

I just carried Fernanda's in my pocket!

I like the idea of carrying it in your pocket - or somewhere else safe.

I think you should just leave it in the US and give it to her later. If it's worth 10k, she probably won't wear it on the streets anyway, and if you end up somehow having to pay duty, it'll be like $9,500. Traveling with this ring strikes me as very unwise.

Well, I guess leaving at home will take the romantic out of proposing to her in Brazil and with her family....that really sucks.

If you didn't bring it, you could buy a nice other band to give her for the proposal. Maybe a Brazilian-style ring? AFAIK, a Brazilian girl is not going to be expecting a 10k diamond (or any diamond), so you won't see any fallen faces.

Or you could return it (I have no idea if that's possible) and get a similar ring in Brazil for less without worrying about customs.

but won't he face the same issue when she comes to the states?

I'ts no longer a gift, it's the woman's personal property. D'oh.
Even if he got it there to give to her up here it would be cheaper and the duty would be less. Although that would be just crazy.

if the value of the ring exceeds the amount allowed in duty free into the states, she could run into the same issue if he bought the ring in brazil, that's my point. she'd have to declare it then too and probably pay a duty on it, would she not?

No such thing for immigrants?

The duty-free exemption only applies to U.S. residents travelling abroad and bringing
merchandise back to the U.S. without having to pay duty.

(I believe people immigrating to the U.S. can bring all their personal possessions
and even monetary instruments up to some enormous limit (like $1,000,000)
without having declare a thing.)

OP, it's up to you what you do. However, I have to say that I agree with Alex - it's not too safe for anyone to wear a ring that is worth 10K in Brazil.

Thanks for eveyone who replied. It just so hard to send and bring things into Brazil even for good intention.