Coal to Diamonds
Might exist already, but if you put a piston facing up, then a blast furnace, then a piston facing down, then put 10 blocks of coal in the input slot of a furnace and 10 blocks of coal in the fuel slot, then put a button on the blast furnace and press it, it will make 10 diamonds. Would work irl by heating up carbon (the coal) and compressing it to crystallize it into diamond.