The Virtue Stones are powerful items, first encountered in Ultima IV. There are eight of them, and each is dedicated to one of the eight Virtues of Britannia. They are also accordingly color-coded to that system. These stones are needed to win the game, because they act literally as keys. Later, the stones showed additional teleporting powers.
In Ultima IV, the Stranger had to find the eight Stones, because they were needed to retrieve the ancient Key of Three Parts (Truth, Love, Courage) from the altar rooms deep within the dungeons. Six of the stones were found in the corresponding dungeons. But the white stone of Spirituality was stolen from Hythloth, and there wasn't a dungeon for Humility.
- The white stone was in a small valley in Serpent's Spine, only reachable with the blink-spell or with the balloon.
- The black stone was found at the Moongate of Moonglow, when both of the moons were in the phase of new moon.
Having done this and obtaining the Key of Three Parts, the stones were again needed, this time to descend deeper into the Abyss. Each stone needed to be used at an altar to descend to the next level, with each level dedicated to one virtue in particular.
The Virtue Stones then vanished for a long time, before re-appearing in the museum in Britain in Ultima VII. A new function of theirs was revealed: the teleport ability. With the spell "mark", a stone could be marked with the position where the spell was cast. With the spell "recall", the spellcaster returned to this position. With the exception of the white and black stones, each stone was pre-marked with the location of the corresponding shrine.
It is unknown what happened to the stones after this.
Serpent Isle Yellow Virtue Stone
In Serpent Isle, there's a yellow Virtue Stone in Flibercio's gold stash. If Nakar's speculation (that it was meant to be a gold nugget) is true, then that may mean that the Ultima developers used a graphical user interface while designing the map.