Socketing System
From Rappelz Wiki
All equipment has at least two available sockets with the exception of two handed weapons. The two handed weapons act like 2 one handed weapons and thus have 4 sockets. The items used to socket the equipment are called Soul Stones. These items can be found in game as drops from monsters and bosses. NPCs called SoulCrafters, located at each major city, are the only ones with the ability to socket items. There are 3 rarity levels of stones, each type giving a special character stat attribute. Each piece of equipment cannot have multiple stones that affect the same attribute. E.g. Only one stone that affects agility. Because 2 handed weapons are like 2 one handed weapons, they can have up to 2 of the same effect provided they do use the same stone rarity.
Soul Stones are level based. They start at 20 and increase in 20 level increments. The three rarity levels are Basic, Lair, and Epic.