Quite hard to determine. Keep in mind that while gold for example is less common then iron it is way easier farmable (pigmen farm) so an experienced player could easily flood the market. Iron while also being farmable via golems needs fairly more time to do so while being also more useful. Emmeralds can be easily generated using a single farmer villager + auto-pumpkin farm. Therefore Diamonds, while mending keeps the demand low, is the most valuabl ime & stable ore
Edit: I do seriously support the idea of using clay. Once given worth .wild would be empty fairly quick so people would have to plunder their local recourses. Since not exactly being the most common building block it would remain quite stable. Contra tho is that at beginning it would be nearly impossible to give it a fix value, quite like bitcoin