For this cake, which was based on a photo of a cake that the bride brought me, I made the brooches. The original cake had real ones on it, but I made gumpaste ones so that the colors would coordinate with the bows.
I used a mold that I had made from a brooch that I own. After it dried overnight, I painted the recesses with gold luster dust mixed with vodka, then I painted the stones with lilac crystallized pearl color.
I used an isomalt stone and attached it to the center of the brooch using a little drop of melted isomalt, and I added pearl dragees to the inner circle design. When you work with the gumpaste and paint it, it will soften up enough that you can push the dragee into it without having to use anything else to adhere them. If you don't trust it to stay put, you can use some isomalt or royal icing to make them stick. By not using anything, though, you avoid the possibility of the "glue" oozing out of the side of the dragee.
To stick these onto the cake, I wet some of the purple fondant and stuck it to the brooch, then to the bow on the cake. You can also make the brooches on skewers so that you can insert them directly into the cake.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com