It won't.
public int getTabIconItemIndex() {
return ItemIds.OBSIDIAN_PEARL;
}
The method I want to use just needs the item ID, hence why I'm passing the ID of my custom item.
Not saying your way is wrong, just saying it's not the way I want to do it.
It should be working, if you look at net.minecraft.creativetab.CreativeTabs; that's how they do it, so I'm a little confused right now