''Too Long Didn't Read'': The higher your skill, the smaller the bonus is from rarity and mining imbue. All the details are here: https://www.wurmpedia.com/index.php/Talk:Mining#Mining_Bonus_Data_Max_Quality
You can use the following equations to find the skill (x) at which you will get a particular max ql of ore (y). Just plug in whatever value for x and solve for y to get the skill you need to get obtain that ql ore or vice versa.
Rare (no imbue): y=0.5861x+41