Virtue Stones

The Virtue Stones are powerful items, first encountered in Ultima IV.

Description
There are eight of these stones, and each one is dedicated to one of the eight Virtues of Britannia. They are also accordingly color-coded to that system, being blue, yellow, red, green, orange, purple, white and black. These stones are needed to finish the original Quest of the Avatar, because they literally act as keys in multiple places. Later on, the stones showed additional teleporting powers when used with the correct spells. The stones appear in Ultima IV and Ultima VII.

Age of Enlightenment
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 opposing the respective virtue. The white stone of Spirituality was stolen from Hythloth, and was instead found in a small valley in Serpent's Spine, only reachable with the Blink spell or with the balloon. It was, somewhat fittingly, the highest accessible spot of the game world. The black stone of Humility was not found in a dungeon either; there was no designated dungeon to specifically oppose Humility. Instead, it could be found at the site of the Moongate of Moonglow, when both of the moons were in the phase of new moon. Thus, it was removed from Britannia in a sense.

After 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 turn the altar into a ladder and descend to the next level, with each level being dedicated to one virtue in particular.

Age of Armageddon
The Virtue Stones then vanished for a long time from Britannian history, before re-appearing in the Royal Museum of Britain at the beginning of the Age of Armageddon. A new function of theirs had been revealed in the meantime: 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. The white stone just led to Britain, the black stone to the courtyard of Castle Britannia.

It is unknown what happened to the stones after that.

Trivia

 * In Ultima IV, each Virtue Stone recovered grants (only) the player's character 200 XP.
 * Humorously, the yellow Virtue Stone of Compassion appears in Ultima VII Part Two, inconspicuously nestled amongst Filbercio's treasure cache. In light of the stone's marked resemblance to a gold nugget or gem, this could either be a tongue-in-cheek easter egg or genuine human error.