The Aquamarine is an item in Everhood. It is obtained in the Junkyard of Mushroom Forest after completing the Jump Rope fight and then opening the dome with Red's arm. The Junkyard is accessible via the locked door on the west side of Mushroom Forest, which can be opened with the Golden Key obtained from Brown Mage's trading quest.
This item is one of three gems required to access the Dev Gnomes fight, alongside the Emerald and the Ruby.
Story
According to the note in Green Mage's playroom, Brown Mage knew of the location of the Aquamarine but decided not to tell Green Mage "for their own good." The gem's location inside the dome in the Junkyard suggests that Professor Orange used it as a power source for the Masterpiece.
Gallery
The interior of the Masterpiece's dome