It is really sweet that you want to buy her a gift for your big day. you need to buy her something that means something to both of you its not about the money its about her waking up on her big day and opening a gift from you that shes gonna go Ahhh her really loves me and bring a tear to her eye. what about a memory box you need to find a really pretty chest type box not too big shoe box size or slightly bigger. And put into it all your memories or things that mean somthing to both of you, like some pictures of a special holiday or receipts for somthing that was special, a picture of where you proposed, or a copy of your song, it all needs to relate to your lives together so far, and then be ready for your future, you could put a note in saying whatever you want like ' lets fill it with some more memories or for the memories were gonna have, the choice is yours, it wont cost too much but will be very personal to the 2 of you. if thats no good then just a plain old bit of jewellry would od but i know what i would prefer.
good luck in june