Wisdom Gems are items in Phantom Hourglass. Link uses these gems in order to channel more power from the Spirit of Wisdom. They are scattered all across the World of the Ocean King, and Link can find a maximum of 20 Wisdom Gems. Upgrading Neri at 10 Wisdom Gems is the equivalent of acquiring the Blue Mail — it reduces Link's damage by half. Upgrading at 20 gems reduces his damage to one quarter like the Red Mail would in previous titles. Also, if Link has a shield, it allows him to stun weaker enemies if they run into it.
Various places throughout the World of the Ocean King
Upgrading the Spirit of Wisdom