Or True Polymorph in 5e.
PAO is one of those spells that can be so useful, yet a campaign destroyer if the DM does not keep a firm hand on it. It isn't making ancient red dragons from pebbles that I find the most worrysome as that has a limited duration. No, it is turning pebbles into gunpowder, uranium, a high explosive or some exotic matter sword that happens to be an allergenic for beholders.
The only way I would allow any of those is if the caster does research or hires a sage to do it. Of course if a sage is hired, they may sell the formula to others, but that is a well known possibility, one that keeps sages abnormally wealthy (either bribes or sales).
Depending on how research is done, this could throw in some adventure hooks as the actual raw materials for the desired qualities may be supernatural and require collection from across the planes, especially the elemental planes. Food for thought.
