From the official Twilight twitter (right click and open in new window to see it in high quality!):

Here’s a close-up of the ring 😉

Aw! Bella’s packing up and getting ready to move out!

[Update]

Here’s a side-by-side comparison of the engagement ring in Eclipse and the one from Breaking Dawn:

They do look different, don’t they? The Breaking Dawn ring looks more round.