Wisdom Gems are the spirit gems that Link uses in order to channel more power from the Spirit of Wisdom. They are scattered all across the overworld and Link will find a maximum of 20 wisdom gems by the end of his quest. Upgrading Neri at 10 wisdom gems is the equivalent of acquiring Blue Mail--it reduces your damage by half. Upgrading at 20 gems reduces to one quarter like Red Mail would in older titles. Also, if you have a shield, it will allow you to stun weaker enemies if they are to run into it.