The Philosopher's Stone is a puzzle element introduced in Legend of Grimrock II, making appearances in various dungeons. They are large, stone blocks emblazoned with the Air Rune on each side.
Philosopher's Stones are always found on special black tiles, which they can be moved over without any difficulty. The tile it's on has to be activated, however, or it will not budge; an active tile glows either blue or orange. Typically, if a tile that glows orange appears, the goal is to move a Stone onto that tile.
A Philosopher's Stone cannot be moved off of the black tiles. If a floor contains both active and inactive tiles, Stones on it cannot be moved onto an inactive tile.