Allen Tucks holding an Energy Stone
An Energy Stone is a mysterious mineral from Star Ocean: The Second Story, which has high energy density capable of emitting heavy radiation. This is enough to cause mutation among humanoids and animals, turning them into ferocious monsters in the very end. The origins of the energy stones are, however, never explained in the game.
It is the cause of Allen Tucks' sudden change of behavior, after one is found in the Salva Mines and he keeps it for himself. He later requests Mr. Bosman to build a secret tunnel for him, leading to a demonic shrine and altar. Later on he attempts to kidnap Rena, forcing her into marrying him in the demonic shrine. Claude succeeds in saving Rena, after defeating Allen in his mutated form. Allen is able to return to his normal self when the energy stone shatters after the ensuing battle. An Energy Stone can also be used for good practices, as the party travel with Leon to collect Energy Stones used to power the secret weapon project, Lacuer Hope.
Energy Stones play an even more important role in the anime Star Ocean EX, as the source of the the sudden appearances of gigantic monsters.
Energy Stones are also necessary for the creation of Quadratic Keys, as such they are highly valuable to developed civilizations with knowledge of Quadratic science.