I misspoke a little. Different tier items have different max levels. You can only get above those max levels with stones. I upgraded my unique past level 100 today and it did not require any stones. 

 

It would be nice if someone could give us the breakdown of which tiers have which max levels.

Ont: By spending. Or just patience! Unique(tier 1) is max upgrade lvl is 120. If you wanna get 121, you need 5 stones of the same element. For example fire > red stone. 5 stones = 5 lvls. 120 > 125.

Ont: By spending. Or just patience! Unique(tier 1) is max upgrade lvl is 120. If you wanna get 121, you need 5 stones of the same element. For example fire > red stone. 5 stones = 5 lvls. 120 > 125.