This is where you buy the diamond before proposing and then shop for the setting together afterwards. It is quite similar to the previous option but completely takes the heat off you to guesstimate what her perfect setting is.
I think this is a great way to do it because you go to a lot of time and effort to find her the perfect diamond and she will appreciate this immensely. You are then also thoughtful enough to let her make the decision herself as to how the diamond is set – a double winner!
Two of my best mates who proposed to their girlfriends prior to me and who I took advice from on diamond engagement rings used this method. It was very well received.
It also ensures that you stick to your budget for the diamond without being influenced by your girlfriend on how much you should be spending.