I just purchased all 13 stones and placed them in my house, the Wraithhouse, but only the Shadow stone doesn't work. I placed it with the same method as with the other stones, it was high-lighted green before locking it in place, there were no obstructions and when my character walks up to it, there's no prompt/indication that its there. With the other stones, you'll see the "X" to activate the stone, but not the Shadow stone, it's like its not there but the character can't walk through it. I've placed the stone in different areas of the house with no change in status, I've placed the stone back in my inventory, then back on the floor, with no change, I've logged out and back in with no change with the stone.
Its only the shadow stone that is giving me problems.