Diocletian was trying to reform the economy of the Roman empire, and feared that if the alchemists succeeded in creating gold, its value would be lost and inflation would ensue.

Don't forget that Newton believed transformation of base metals to gold to be possible. Under the ancient system of four elements, every substance* was made up of a mixture of Earth, Air, Fire and Water where these were the 'ideal' versions of earth, fire, air and water. It should therefore be possible to rearrange the components to change one metal to another.

Newton rejected this system, but still believed that the fundamental elements were few, so the same argument applied. It was only via a 300 year diversion, through the 100-odd chemical elements, that Newton's few fundamental particles came to be known as quarks, and that no simple chemical reaction had the energy to provide the desired transformation.

As for Brother Lemert and Moncure Conway, I expect these are earlier or later alchemists, who are still around today, and still can't get their experiments to work except in a spiritual sort of way.

(*word used in the modern sense.)