Amethyst

Although your limbs feel heavy as lead and your brain demands rest, you crouch by the lantern and fumble to pick it up. Carefully, you inspect it. The rock is semi-transparent with a weird blue cast, and glows softly. Counterintuitively, the closer to the light you come the drowsier you feel. You shake your head, too confused and sleepy to think hard. You'll ask Hyri in the morning.