From superman.nu: Supermanica
Amnesium is a rare space metal. The rays of this material make those affected forget recent memories.
Jimmy Olsen holds a jar of this material while looking at Superboy's mineral collection when he is sent by mistake to the past. Superboy then sends him back to the future holding the jar, making him forget the whole trip. (SB No. 55, Mar 1957: "A Visit from Superman's Pal").