Runes are used to imbue equipment with special powers. Use Runes to customize your gear, depending on your combat style or class. You can't equip Runes that are of a higher grade than the equipment. There are five types of Runes: Crystal, Ruby, Sapphire, Emerald, and Diamond. They are stronger based on their grades.
There are two categories of rune: Attack Rune, and Defense rune. Attack runes have offensive bonuses, like increased Magic Crit, or Physical Accuracy, while Defense runes can increase Health or boost evasion.
Fusing Runes & Gems
You can fuse runes by clicking on the Equip Rune icon at the bottom of the Inventory UI. You can fuse 5 Runes or Star Jewels to obtain 1 new Rune or Star Jewel. You can try changing options of Runes and Star Jewels, and if you are lucky, you may obtain a higher level Rune or Star Jewel.
The higher the level of the Runes and Star Jewels you use, the higher the chance you'll get a higher level one, and the fee to fuse depends on the level of the Runes and Star Jewels.