The fifth Mythical Crystal is the Lava Crystal, which can be extracted from a lava rock. Though lava rocks are scarce, you might be able to find one on the edge of Tanglevine Jungle near the Mt. Kaklakala Volcano. Go there now and find the rock!
Note: This is a Gold Explorer mission. You must complete Mine the Crystal Catacombs, and Mythical Crystals Parts 1, 2, 3 & 4 in order to complete this mission.
Mount Kaklakala Volcano
