Moonstones

The moonstones are very, very old artifacts. It is unknown from where they actually come, but they are bound to the phases of Britannia's twin moons, Trammel and Felucca. There are eight in existence, each representing a different moon phase. When a moonstone is buried in the ground, a blue moongate appears in that location at night, linked to the phase the moonstone represents. Each moonstone can be excavated and re-planted elsewhere, if the terrain is clear.

Although the moongates they created have existed since Ultima III, awareness of the actual moonstones first came about in Ultima V, when it was learned they could be excavated at the sites of the moongates and re-buried. They were stolen by the gargoyles in Ultima VI and had to be found. While re-buriable, they were essential in combination with the Vortex Cube to return the Codex of Ultimate Wisdom to the Ethereal Void. After the destruction of the Sphere Generator in Ultima VII, they stopped working.

In Savage Empire, the energies of a corrupted moonstone found in Eodon were responsible for the advanced technology of Kotl, as well as the creation of the Myrmidex. The moonstone had to be destroyed by the Avatar.

Trivia

 * While never mentioned in the games themselves, Richard Garriott raised the concept of the moonstones, as well as the Orb of the Moons, being formed out of blackrock in his interview with Caroline Spector for Ultima: The Avatar Adventures.
 * In Ultima Underworld I and Ultima Underworld II, spheres called "moonstones" are also found. However, they are not the same as the eight moonstones of the eight moongates.