TL;DR
For centuries alchemists tried to turn cheap metal into gold and failed. A fusion start-up, Marathon Fusion, now says it has done it, converting mercury into stable gold with the neutrons a fusion reaction throws off, but it will not say how much it made, so nobody outside the company can check. The reason to care is money, not jewelry: if a fusion power plant could sell gold and medical isotopes on the side, the company's own unconfirmed math says the plant could earn a little more than twice what it earns from electricity alone. A second paycheck like that could make the first fusion plants easier to finance, and clean, abundant power could arrive sooner. None of that is shown yet, and no fusion machine has put electricity on a grid.
Below the line, for paid subscribers: The full story: where it stands, how we got here and what's dated next; our Odds board on 5 milestones, starting with "Gold or medical isotopes are sold as a by-product of a fusion power plant by 2040"; our labeled speculation on what it adds up to; and every source.