Hello.
Upgrading has been a concern of majority of the playerbase for quite a long time now, and people seem to have the wrong idea regarding the whole system, so let me clarify some points.
:Blue stone increases the armor grade by 1, this is 100% chance of happening from +0 to +4, meaning after +4 you will have decreased chance of success, there the "pick one from 3 cards" minigame starts.
Same goes for Red stone, which increases a weapon grade by +1, this is 100% success until +6, same game starts once you hit +6.
Purple stone can increase the grade by +1 / +2 / +3 on success, this is 33.3% chance for all three, purple stone is for armors.
Green stone is the same as purple, but it's for armors.
Black-Green stone decreases an armor grade by 1, 100% success rate on every grade.
Black-Red stone decreases a weapon grade by 1, 100% success rate on every grade.
Failing will result in -1 downgrade if you did not use a white stone, regardless if you use normal stones (red-blue) or advanced stones (purple-green)
Failing will result in no penalty if you did use a white stone.
Broken will result in reseting your item to +0 if you did not use a white stone.
Broken will result in downgrading your item if you did use a white stone-
Now, people seem to have the wrong idea on how white stones work, some people claim that White stone decreases the success chance, which is false, it has been confirmed by Daneos that White stone does NOT affect the upgrading.
When you use a white stone, there is no penalty if you get "Failed", and there is a -2 downgrade if you get broken.
Some people also claim that you cannot get success by clicking the right card, which is also wrong, because I myself got +15 only by clicking the right card, every single time. (i should have made a gif, i know, kek.)
The minigame was coded in a way to show you "success" on either left or middle if you get failed or broken, it will never show success on the right side (unless you actually get success.)
People also claim to have 1/3 chance of upgrading at any grade, which is also wrong, here is the official percentage list provided by a Hardlock
Now let's say you have a +10 weapon, it means that you have 10% chance to get a success, also a 33.33% (because purple stone can give +1 or +2 or +3) chance to get +13.
[Weapon]
Grade 0-6: 100%
Grade 6-7: 30%
Grade 7-8: 25%
Grade 8-9: 20%
Grade 9-10: 15%
Grade 10-11: 10%
Grade 11-13: 5%
Grade 13-14: 4%
Grade 14-15: 2.5%
[Armor]
Grade 0-4: 100%
Grade 4-6: 60%
Grade 6-8: 40%
Grade 8-9: 30%
Grade 9-10: 20%
Grade 10-11: 10%
Grade 11-13: 5%
Grade 13-14: 4%
Grade 14-15: 3%
When you have 10% chance, you can count it as a ticket system in lottery. Let's say you have 100 tickets, and only the first 10 tickets are the winners, and the machine will randomly generate a ticket number from 1 to 100, this is the "basic" way to understand this, because some people even claimed to have found a "bug to upgrade".
Clicking a card will just generate a number, it doesn't matter if you keep clicking the same or change everytime.
Most of the players who already have +11-12 gears probably knows this, this thread was not made for you. This thread was made solely to clear up some wrong """facts""" such as having 1/3 chance everytime, or the right card never giving success.