Hmm, this may work if you let each individual layer completely cure before pouring the next one? However, you're bounded by 'gravity', if that makes sense--you would pour a partial color into the gem mold, and it would fill the bottom. Then the next layer would fill it and they would go from the 'front' to the 'back' of the jewel. This might look neat for spherical gems, though you might get a 'muddy' color looking through the first color because it would have every color behind it (and form kind of a multi-color brown). But, it would allow you to have 'rings' of color coming out from the center! Layering the colors wouldn't work with any other shape, methinks.