Straight-edged rocks work best for this--they give it a real jewel look. I can easily find rocks of this sort in my kids pockets, closets, on their bookshelves...you get the point.

We pulled out a few metallic and pearlized paints. My children were pretty excited since these are my paints.

My two-year-old was actually very good at this. He was really set on covering each rock completely.

Our gems turned out so beautifully and inspired some wonderful imaginative pirate play!