The higher the quality of the equipment the more often the item can be improved. A normal piece of equipment can be improved 3 times; a rare piece of equipment can be improved 8 times; you can improve an epic 12 times and a legendary 16 times. A unique item (found only in a special event called the “Blade Coast”) can be upgraded 20 times.
With each improvement on the same item the chance that the improvement is successful decreases. The first improvement has a 100% chance of success, while the second has a success rate of 86%.
If the improvement was unsuccessful you will only lose the coins that you paid for it, the item will remain as it was before, and will not lose any improvement that had been achieved before.
By using rubies it is possible to increase the chance that the improvement is successful to 100%.