IndyuceUsing MMOItems API you may register custom stats as complex as abilities or item restrictions. The first thing you will need to do is define a class which extends DoubleStat which is an abstract class with many functions, we will go over what each does, but first let's deal with the ItemStat constructor you need to call.
super("GEM_SOCKETS", new ItemStack(Material.EMERALD), "Gem Sockets", new String[] { "The amount of gem", "sockets your weapon has." }, new String[] { "piercing", "slashing", "blunt", "offhand", "range", "tool", "armor", "accessory" });