Thats pretty cute. If I may, I dont think you need to worry about "having to have a rock". I think she's going to be over the moon about marrying you, diamond or not. When I got married, we both got plain gold bands with an engraving and I was happy with that. Our jeweler told us though that some girls go in there and they're a nightmare, practically asking for your entire life savings. Your girlfriend sounds sweet and down to earth though, so I think shes really going to find it cute what you're doing. And ofc later you can have her pick out the 'serious' ring, but I dont think shes going to be pissed if its not there during the proposal.
I'm not sure what substance you'd use to shape the diamond, but I would suggest checking etsy for candy molds that are gem-shaped. These are usually made from silicon and come with a lot of different 'cuts' and sizes.