I did not find anyone who reported this, but the Stone of Seeking has the wrong shape.
The crystal I got a few moments ago had the shape of a Weapon crystal, but has to be put in an Armor crystal socket.
Stone of Seeking.bmp
The left xtal = The Fangs of Fury
The Middle = The Stone of Seeking
The right xtal = Radiant Invulnerable Armor Crystal
It works, but it looks strange.