Wisdom Gem
Wisdom Gems are collectibles in The Legend of Zelda: Phantom Hourglass.
There are 20 in total and are found all over the Great Sea. Once you obtain ten, the Spirit of Wisdom gains power and a new ability can be used. Gaining all 20 increases the Spirit's power further.
For the spirit of Wisdom, getting 10 gems lets Link's defense double. Getting 20 quadruples the defense.