A Moonstone is an item that once allowed players to travel between the Facets Trammel and Felucca before linked Moongates between the facets existed, shortly before the release of Ultima Online: Renaissance. Shortly afterward the release of UO:Renaissance, this functionality was deactivated and today they are just historical curios. They were primarily used for players to explore the new Trammel facet, even before housing was activated there, as well as return to Felucca.
Moonstones were found as random loot on some monsters and led to the opposite facet from which they were found. For example, a Felucca moonstone would only be found as monster loot while in Trammel. Moonstones no longer spawn and are found only in player possession. They were named after the two moons of Sosaria.
Moonstones were used by double-clicking them in the character's backpack. If the character was in the facet opposite the moonstone's type, the moonstone would drop to the ground and slowly begin sinking into the ground at the character's feet. After the moonstone had sunk from view, a temporary gate to the exact same location on the opposite facet would appear for a short time, roughly 30 seconds. They generally could not be used while the player was flagged for combat or in a guardzone. Once used in this manner, the moonstone was consumed. If a player changed their mind, they could quickly pickup the moonstone before it had sunk all the way into the ground to save it.
Another interesting feature of moonstones was by single-clicking them, they would change color to reflect the phase of their respective moon. Another way of determining Moon Phase, still functional today, is to use a Spyglass.
Moonstones existed in the single-player Ultima series of games, and were thought to have been made of Blackrock. For example, in Ultima V: Warriors of Destiny, a moonstone could be used to temporarily open a gate to a town.