Let me first say that for this purpose, isomalt is probably the better choice to use, as opposed to real sugar. Since you only need a small amount at a time, you'll need to be able to melt a little bit at a time, and isomalt is less fussy when you re-melt it. I've done these with real sugar, but you have to be a little more observant.
silicone mold that can withstand the heat of the hot isomalt. Make sure that it isn't too rigid. If it's flexible it will be easier to get the brooch out of the mold.
Finally, you can use a solid piece of gumpaste, then paint it with a metallic color. I've seen a lot of cakes that have either gold or silver gems on them, and they're gumpaste with silver and gold luster dust paint. I'd say that this is my least favorite way to do them, since by comparison, the gems aren't as shiny. If you made the gems one color and the backing gold that might be better, but I prefer the look of the isomalt or sugar jewels.
All of the molds pictured here , along with others that aren't used in this demoavailable in my Etsy shop in the molds section, are
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com