Wisdom Gem
Wisdom Gems are collectibles in 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.