One of the rarest and most expensive substances on Earth isn’t gold — it’s californium-252, and it can cost around $27 million per gram. Californium-252 is a synthetic radioactive element produced in specialized nuclear reactors. It does not occur naturally in usable quantities and must be created atom by atom through prolonged neutron bombardment of other heavy elements — a process that can take years. Its extreme price comes from rarity and utility. Californium-252 emits powerful neutrons, mak