Rock skin was an early combat spell. It became available at level 11 GoodMagic, and consumed 3 earth runes, and 1 mind rune when cast. When cast, it would increase your Defence by 10 levels. When the Magic and Prayer skills were combined, it was taken from the good spellbook and added to the newly formed Prayer book. The new Prayer version of the spell would have a similar effect, but at the cost of prayer points instead of runes and with a requirement of level 10 Prayer.
This information has been compiled as part of the update history project. Some updates may not be included - see here for how to help out!