Hi! Are there any suggestions of what I could use to make the jewels Starfire has on her gloves and collar?
http://images.titansgo.net/screenshots/310/135.jpg
I'm not too familiar with building materials, but I would think that any standard costume sculpting material, like fiberglass or plaster-of-paris, would work. Probably whatever is lightest so that it stays in the costume easier. Since it's a pretty small and simple shape, it shouldn't be much trouble no matter what you do.
Now, if you want them to flash with the red "Trouble!" alarm, that's a different story...
1) Your image gets blocked for hotlinking. Now that I've found a picture, though ...
2) I'd suggest resincasting, actually. That's one of the most common (and easiest) ways to do gems, particularly round ones, and it's fairly simple -- plus, there are a number of people out there who either sell them premade or take commissions for them if you don't want to do your own. I did these suckers (http://i173.photobucket.com/albums/w77/myalchod/Misc%20Refs/Picture082.jpg) in a fit of experimental boredom one night playing with a mold and a paint tray, for example.