Enhancement crystals have a single function in Rise Online: to improve item stats.
An item can be evolved with infinite enhancement crystals, and if the enhancement succeeds, it will add +10 to both Defense and Attack. However, with each enhancement stone used, the difficulty for the next enhancement crystal to succeed will be doubled.
Let's say an enhancement crystal has a 2/4 chance of succeeding, i.e., it has a 50% chance. On the second crystal that a player applies to an item, the chance of success will decrease to half that of the first crystal. In other words, 25%. Consequently, the third enhancement crystal will only have a 12.5% chance of success. The fourth will have 6.25%. The fifth, is 3.125%.
While the chance of the next success drops by half with each successful enhancement, it is most common to see weapons with only +4 Enhancement, at most.