There is a comic book series called "Concrete" in which the main character is a human whose brain was placed in an alien body. The body is silicon based and resembles concrete.
He mainly eats things like cement or rocks and his waste is removed by the outer layer of his body gradually flaking off. Basically, he doesn't excrete anything, he just leaves alot of sand-like dandruff on the ground.
I suppose Krans could work in the same way, the dead outer layer flakes off gradually and is replaced from within by the food they consume. That way any chips or gashes they get during their lives would go away when they shed their skin or whatever. Maybe little bits of gold dust or rust would be mixed in with the sand, not sure what a kran would get by eating diamonds though.
Perhaps diamonds are a product of krans eating stuff they shouldn't. Get too much carbon in your diet and it all collects in your appendix and is slowly compressed and heated into a diamond. After a few years it gets pushed out of the body and exits the skin like a lump or a sore. Sort of like how pearls are with oysters.
Anyway, that's one theory but there is no evidence that this may or may not be the case, Kran physiology shall puzzle scientists for ages I guess.
