Diamond Doors are usable pieces of furniture that can be crafted in your Crafting Menu (opened by "Q" as the default key) or at Crafting Tables (that still exist in older worlds), but only after the crafting-recipe for Diamond Doors has been unlocked by taking/crafting an Iron Bar Door for the first time, which in turn is unlocked by taking/crafting an Obsidian Door. To craft 1 Diamond Door, you'll need (as of R29 in March 2016): Currently (as of R30 in April 2016), other players can open and close doors when visiting your world and/or claim, even if they are set to "Visitor".