1) The stone is lodged in the wall of the north building adjoining the square where you fight the first Nhoss.
IVORY CITADEL (LOSTLIGHT)
1) You can find the stone while going through the western part of the Citadel. The secret you're looking for is hidden inside the main tower with the yellow ball, which has to be charged in the course of the Citadel's main quest.
2) The stone can be found after unlocking access to the north door of the central area with the large waterfall and taking the path leading to the Scribe. Reach the balcony shown on screenshot 1, using two nearby portals. Turn left and shoot at the portal shown on screenshot 2.
The only thing left is to enter the room with the portal mentioned earlier and shoot the secret.