This is pathetic. You accepted an engagement proposal. Love, trust, honour was presumably involved in some degree. Now, eight weeks later, you are on a public forum asking whether it is 'ok' to keep the expensive ring when you dump the guy.
The fact that you are even asking for guidance (read: permission) appears to illustrate your gargantuan lack of moral fiber.
To give you the benefit of the doubt you could be one of two things...
1- a selfish-uncaring-lying-thief
2- too stupid to understand why it is wrong (in which case your IQ would have to be less than 65)
Which one are you?
Doing the 'right thing' in the end does not make you better. You were looking for justification to steal and didn't get the support you hoped for. If the value of the item was higher, it would probably have tipped the scale the other way.
I can't believe all of the posters fucking cheering you and congratulating you for not stealing from someone. Someone who will be devastated by the break up and vulnerable. Someone you claim you wanted to marry or, at the very least, continue to have as a friend. Way to go, Champ! Whoo hoo!
I think I'll go puke now.