Moonstones

Moonstones are extraterrestrial stone artifacts connected to the workings of Moongates. They appear in  Ultima V and  Ultima VI, as well as in the Underworld Series and the  Savage Empire. The Orbs of the Moons share some characteristics with moonstones, and are actually referred to as "moonstones" at times.

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 Lord 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 teleportation 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 harnessed 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.

The Ultima 6 Project
In The Ultima 6 Project it thankfully is - unlike the original - impossible to re-bury the moonstones and thus create plotstoppers. However, the moonstones are needed to repair the eight city moongates, enabling them to work even without the stones being buried in the soil. Without that step, the Orb of the Moons won't accept the city gates as valid destinations.

Trivia

 * While their origin and composition is never specified in the games themselves, Richard Garriott indicated that the moonstones, as well as the Orbs of the Moons, are formed out of blackrock in his interview with Caroline Spector for Ultima: The Avatar Adventures. Ultima VII: The Black Gate art director Jeff Dee, meanwhile, submitted an unsuccessful project proposal for a Worlds of Ultima-style game, in which the player would travel by some means to Britannia's twin moons, procure replacement moonstones, and reactivate the moongates following their destruction by the Guardian's Sphere Generator.
 * 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.
 * The moonstones can be dug up in Ultima V, but that's not a smart thing to do.
 * In a fit of insanity, the Avatar can bury a moonstone in a dungeon in Ultima VI, ensuring it can never be dug up again.