Moonstones

Moonstones are stone artifacts connected to the workings of moongates. They appear in Ultima V and Ultima as well as in the Ultima Underworld series and in Savage Empire.

Description
The eight moongates of Britannia are tied to eight moonstones, which lie beneath the locations where the portals appear at regular intervals of the world's lunar cycle. Each moonstone is marked with an image depicting one of the eight phases of the moons. Should the moonstones ever be uprooted from the soil (as they were by Draxinusom's troops during the Gargoyle War) the gates associated with them will appear at the location where they are re-buried.

Moonstones also appear to have mystical properties outside of their connection to the moongates. During the last days of the Age of Enlightenment, they were required, in conjunction with the Vortex Cube, as a vital part of the ritual the Avatar performed to send the Codex of Ultimate Wisdom into the Void. In runic magic, small moonstones (not related to the major eight) are employed by magicians as a means of teleport via the Gate Travel spell.

Giant Moonstone of Eodon
In the early history of the realm of Earth a meteorite composed of the same material as the Britannian moonstones struck the planet. This great black stone was eventually harassed by the great Kotl civilization, who used its energies to power the technology of their empire and to craft a servitor race known as the Myrmidex.

In time, however, the ant-like Myrmidex revolted against their masters, altering the giant moonstone such that it allowed them to destroy the Kotl. This act caused the artifact to place the valley that they inhabited in a temporally different state than the rest of the Earth, cutting the Myrmidex and the scattered human slave stocks of the Kotl off from the rest of civilization. Centuries later, the Avatar would destroy this giant moonstone on behalf of the united tribes of Eodon, and in doing so would precipitate the fall of the Myrmidex race.

Trivia

 * While it is never mentioned in the games themselves, Richard Garriott implied that the moonstones, as well as the Orb of the Moons, were formed out of blackrock in his interview with Caroline Spector for Ultima: The Avatar Adventures.
 * In the original release of Lord of Ultima, players could create moonstones with a high level mage's tower, and collecting these moonstones would later determined the level and capabilities of the player. In the "Ascension" expansion for the game, however, moonstones were functionally replaced with purified resources.