I will add something to what StealMySoda said.
While adding a magic stone (obviously the green ones) u have
a. A chance that the stone works and u get the blue attribute on ur weapon/equip/accesory. You might get only str1 increase (if its a str stone) or u might get the full str increase depending on ur items degree.
b. The stone fails. Nothing will happen to ur item, just the stone fails
c. The stone works but the stat u wanted to change remains the same (includes both magic and attribute stones) and u have a chance of getting assimilation on an attribute.
d. The stone works but brings down the stat u wanted to improve
e. The stone works but brings down the stat u wanted to improve AND messes up another stat (oh yeah thats me

)
Every time a stone works, there is a chance it will change another attribute on ur item through assimilation. The stones with 5% or 35% assimilation can only be obtained from a destroyed item (using destroy rondo to break items).
Creating a stone using tab and elements can give u stones with attribute assimilation 10-20-30%.
Now, if assimilation occurs when the stone works, it doesnt mean that it will bring down a white/blue stat. Sometimes it makes a stat go up (happened to me 2 times) but most of the times i get zeroes ^^
Always remember than if u gonna go past +3 on an item, pour steady and immortal if u dont want to loose it. After +5 if u get a fail , both astral and immortal can dissapear, or astral and steady.