These faux gemstones are intended to look like cabochon stones in whatever setting you desire. They have a sort of medieval flair to them. As such, I have used them in fantasy costuming, like the wizard costumes my family wore to Harry Potter book releases or Halloween costumes. However, they are a flexible embellishment that can be adapted for home decor and jewelry.
I have instructions for other fantasy craft items and examples of items with these gems in them posted on my crafting blog Craftastic World.
I also have a flickr photo group with pictures of these in use, if you need to be inspired. You can join it if you'd like and post your own creations: Faux Gemstones Photo Group
Step 1: Gathering Materials
Clear or colored flat glass marbles
Permanent markers in colors that you like (I prefer Sharpies)
Clay tools for your polymer clay
A glass pan (for baking the finished piece)
Step 2: Making It Sparkle
Tear off a piece of foil that is more than large enough to cover the bottom of the stone. You can use smooth foil or crinkle it for an interesting effect. I prefer it crinkled because it allows for more variations in color. If you want a crinkled effect, wad it up several times first. Smooth out the foil or, to get really deep creases, press down on the foil to flatten it. This retains the deep creases and prevents the foil from getting holes when it is over-worked.