When Mining and using Equipment that boost the Skill level of a Trade skill, such as geology.
You must make your Level match up to that which you want to gain skill from.
Such as if i want to mine Garnet which is a lvl 20 Gem *5 so level 100.
I would want my total geology with bonuses to be at or close to 100 for optimal gains.
So if i have 30 natural Geology skill and i have +200 bonus from items,
totalling 230 i wont be capable of getting skill gains from The Garnet.