It's been suggested before, but it can't be suggested enough. I definitely support this.
I'm not too interested in players not being able to teleport in. Players that want to grief could wreak havoc with a high end pickaxe or rocket IV's anyway. (The game could have used an option to make maps read-only to people that you don't trust, so that they wouldn't be able to destroy tiles or take items from chests.)
But I'd love to make underground habitats that are safe from wraiths and anything else that floats through tiles.
Stone, Luminite and Ectoplasm sounds like a nice recipe.
The Luminite would make it an end-game and costly thing, so people don't have to worry about it being overpowered (which it really isn't anyway).