I decided to make a cake for my friend's daughter and she wanted a Prada bag with gems on it. I have not tried to do these yet. If I have a gem mold do I:
1) can I take pre-existing candies, melt them and pour them in the molds?
2) do I have to follow the hard candy recipe?
3) can I make gems out of fondant/gumpaste and dust them with something?
4) can I make chocolate gems and dust them with something?

I am in a massive brain block and need help! Has anyone tried these things or have a better solution?

I don't know but I would live to know the answer.

If you could melt existing hard candies that would be a great option. I have alot of that available around the house.

1) can I take pre-existing candies, melt them and pour them in the molds?

-Yes you may but they might get sitcky.

2) do I have to follow the hard candy recipe?

-I personally prefer you make it with isomalt and water.

3) can I make gems out of fondant/gumpaste and dust them with something?

-Definitely.

4) can I make chocolate gems and dust them with something?

-You can use luster dusts, platinum dusts or any FDA approved dusts.

If you want real looking gems, I suggest you do #2.

I tried the melting the candy and it didn't work out so well. VERY sticky - not usable (I Used jolly ranchers hard candy)

If you use Isomalt - take it to 370 before using it - it is supposed to stay clearer longer that way.

Also, brush the gems with vegetable OIL - that helps them retain their clearness and shine.

GSA now sells them already made - not sure if you know that or not - I got an email from them the other day with the gems in it.

Here is a cake with the ones I made on it - I didn't do so well (after several tries) I did ask afterwards though and found the info I gave you above!
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1467595

I was going to tell you about GlobalSugarArt also.

If you are not going for the real gem look, you can definitive make them with gum paste, here's a pic of the ones I made!
http://media.cakecentral.com/modules/coppermine/albums/userpics/687815/normal_1253031209.jpg

Awesome guys! You certainly have given me a lot to think about. Will definitely try a couple out! Thanks for being lifesavers!