hmm don't forget Krans are rocklike to us. They aren't actually moving rocks

swords and axes do LESS damage. Thats a whole lot different from NO damage. It just takes a little longer to cut them open then when cutting a human.
As for the creating of Krans the setting says it was done by talad so they didn't evolve. They were magicly created so if something seems impossible it's godly powers in this case

How a Kran basicly works would be good to know though for those who roleplay one. What would happen if a ran get wounded. What would come out, how would a fatal wound look like? Where are his organs and what place would be their equavalant of a stab through the heart?