Quote (Elativ93 @ Oct 9 2021 10:00pm)
1 Perfect Skull + 1 Rare Item + Stone of Jordan = 1 High Quality New Rare Item of the same type
This formula has the possibility of creating a higher quality Rare Item than the following formula. ilvl = int(.66 * clvl) + int(.66 * ilvl) That is, the ilvl will be equal to .66 times your clvl (rounded down) plus .66 times the input item's ilvl (also rounded down).
Diadem can always roll any stats regardless of their quality level dropping though so you should always reroll them with 6 pskulls.
That being said, gg diadem.