When I was looking for faux gemstones for costuming, the choices were very limited. The colors were also boring and didn't have any of the natural variations I was looking for. So I made them myself out of a few basic craft materials.
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