A single stage pokemon is one that doesn't evolve. Sableye is an interesting looking thing. It's eyes are made of gems, as a result of it's only eating minerals and rocks. It lives in small, dark caves, and the gem-eyes help it see in very little light.
Once again, I'm thinking about the way in which explanations are provided for the strange physical features of pokemon.
Biologically, I don't think a creature could have rocks for eyes, but the reasoning behind the need to refract light sort of makes sense.