Moon Stone (暗黒石 ankokuseki "Darkness Stone"?) is a Key Item in Chrono Trigger. It is obtained by defeating the Son of the Sun in the Sun Palace (Not to be confused with the Sun Shrine where the Moon Stone is charged) during the Future.
Storyline
The Sun Stone was used as the primary energy source by the Kingdom of Zeal before the advent of Lavos and the Mammon Machine. When it runs out of energy, it becomes a Moon Stone. It can be turned back into the Sun Stone, but it must be left in the sun for a period of over 65 million years.
It was briefly stolen in the year 1000 A.D. by an explorer who delivered it to the Porre Mayor, who kept it concealed. After some changes in time, the originally greedy mayor became a jolly man who surrenders the stone to the party without a hitch.