Ether Gem

The Ether Gem is a magical artifact capable of trapping spirits -- such as daemons -- within itself. It appeared in  Ultima VII's expansion pack Forge of Virtue.

Description and Abilities
A small, brilliantly blue jewel, once a spirit is bound within the gem, it becomes subject to the will of whomever holds it. Its master may partake directly of the mystical energies of whatever is bound within it, thereby replenishing their capacity to cast magical spells.

History
The Ether Gem originally belonged to the magician Erethian who took the artifact with him to the submerged Isle of Fire in the course of his studies. Arcadion, a daemon bound into the  mage's service, mistakenly believed that this jewel could free him from his imprisonment in a magical mirror, and grew increasingly eager to possess it - however, when the  dragon Dracothraxus stole this artifact from the scholar, she turned a deaf ear to Arcadion's pleas for aid, leaving both the daemon and its master eager for her death.

Eventually, the Avatar arrived on the island. Finding that Dracothraxus served as guardian to the isle's Test of Courage, the hero vied against the beast in combat, but found, despite their best efforts, that she would not die. The dragon advised the hero that the weapon which would slay her was not yet forged, and gave the Avatar the Ether Gem as a reward for their valor. The hero thereafter brought the jewel to Arcadion's mirror and shattered it, leaving the daemon trapped within the bauble's confines and subject to the Avatar's mastery. Determined to best Dracothraxus, the hero turned to Erethrian, who assisted them in forging a sword made of  blackrock. To balance and complete this odd piece of weaponry, the Avatar bade Arcadion bind himself and the Ether Gem into the hilt of the blade, thereby imbuing the black sword with his power. Armed with such a weapon, the hero set out once more to face the dragon, and succeeded at last in slaying her.

When Arcadion escaped the gem in  Ultima VII Part Two - being in this foreign land had weakened its hold and him and the Avatar had no choice but to let him go in the end - it became damaged and had to be repaired with the Flux Analyzer before it could again catch spirits. It was later used to great effect to catch the Banes of Chaos, acting as temporary prison before putting them into Soul Prisms.

Trivia

 * In Ultima VII Part Two, there are three more portraits of the Black Sword, each with a differently animated Ether Gem. Since all of these have the gem in verious shades of red, it's most likely that the Avatar was meant to converse with the Banes of Chaos while they were trapped inside the gem.