CRYSTAL SPIRE (LOSTLIGHT)
1) The stone is hidden on the top level. After using the lift to get to the balconies, turn left and explore the left walls as you go forward.
1) You can find the stone in the tunnel soon after defeating the first Nhoss. Examine the ceiling of the right branch.
2) You can find the stone while going through the ruins after finding the third piece of the Rod of Arafel. It's in the same room as the Fists of Elhazar.
IVORY CITADEL (LOSTLIGHT)
1) The stone can be found in the main area of the Citadel, the one with the large waterfall. The secret is attached to the upper part of the statue (screenshot 1) and to shoot it you'll need to reach the balcony on the third floor of the structure (after restoring water to the eastern and western parts of the Citadel).
2) You can find the stone while going through the northern part of the Citadel. Get to the room with two rotating devices. Stop at the second one (the one near the inactive portals) and aim at the secret shown on screenshot 1.