1) The stone is above the mausoleum door (opened with the Bone Key received in return for collecting a specified number of the Book of the Dead pages).
2) The stone is on the statue of an angel, left of the entrance to the Crystal Spire.
3) The stone is on the statue of an angel, right of the entrance to the Crystal Spire.
1) You can find the stone on the wall of the north building adjoining the square where you begin the mission on Earth.
2) The stone is lying on the pipes below the ceiling in the room where you collect the first piece of the Rod of Arafel.
3) You can find the stone not far from the spot where you obtain the grenade launcher, right after exiting the tunnel. It's in one of the windows of the building on the right.
4) The stone can be found while going through the bottom part of the long bridge, soon after taking care of the second Nhoss and on the way to the third piece of the Rod. Look around for broken fragments of the bridge. The secret is attached to a pillar.
IVORY CITADEL (LOSTLIGHT)
1) You can find the stone while going through the eastern part of the Citadel, in the initial stages of this location's quest. Get to the wall fragment shown on the above screen.
Wall-run along the left wall and turn left. The stone is lying on a window.
2) You can find the stone while going through the eastern part of the Citadel, right after destroying the yellow crystals with shadowbombs and reaching a tower with two inactive portals inside. Stop at the stairs leading up and examine the arch above them.