The Magma Orb is an item in Golden Sun: Dark Dawn. While vital to the plot, it is never formally obtained by the party.
The Tuaparang commanders Blados and Chalis manipulate Matthew's party into a battle with the Mountain Roc with an additional threat that the Sanan princess Hou Ju would be boiled alive if the Magma Orb was not retrieved. After defeating the Mountain Roc, they find the Magma Orb within the creature's belly. Having learned previously of the dangers of the Alchemy Dynamo, the Adepts attempt to leave it behind. However, Ryu Kou, Hou Ju's brother, was traveling with the Adepts and took the Magma Orb for himself. Eventually, Ryu Kou would be manipulated into using the Magma Orb to activate the Alchemy Dynamo, causing the Grave Eclipse.
Note that the Magma Orb can be obtained by the party if you use cheat codes. If you choose to trigger the cutscene with Ryu Kou, the Magma Orb will disappear from your inventory. If you do not trigger the cutscene and leave Talon Peak to continue the main story, the Magma Orb is a permanent addition to your inventory. As it was never meant to be obtained, it cannot be sold or removed from your inventory.
The Magma Orb's description should you choose to obtain it:
A sphere of solid lava, glowing with power.