An often-recurring plot point in older Superman comics is that he can create diamonds from coal with his bare hands, but doesn’t do so on any large scale because it would depress the price of diamonds. Economics 101 ftw! In one comic the villainous plot is to have Superman demonstrate his ability, then destroy the diamonds, and the villains swap the newly-created diamonds for glass which Superman then destroys. (I forget how they are foiled, but never mind. Something to do with the flag of Krypton. It didn’t make much sense, but there you go.)
Ok, fair enough, he clearly could depress the price of diamonds, but – you say that like it’s a bad thing. Just for starters, consider all the industrial uses that would get so much cheaper! Jewelry is really the tip of the iceberg. But if we’re considering industrial uses, then hooking Superman up to a giant hamster wheel and having him generate cheap electricity, or using him to drill for oil, are probably more efficient economically. No, the real goodness is to be had from stopping the diamond wars in Africa. Drop the price of diamonds close to that of coal, and a round three dozen nasty little warlords are out of business, a good hundred thousand deaths per year prevented, and any number of mines using slave labour close down. All in a week’s work for Superman! Alas, it seems nobody at DC comics took Economics 102 or Sociology 101.