Ether Gem

The Ether Gem is a gem capable of trapping spirits, such as Daemons. This is why it is so valuable, especially to mages.

Erethian originally possessed the Gem, but Dracothraxus stole it from him (Erethian was thinking about getting it back, but nothing came from it). In Ultima VII, the Avatar retrieved it from Dracothraxus after defeating her for the first time and used it to bind Arcadion to his/her will, fusing it into the hilt of The Black Sword.

It stayed in the hilt of the sword, but was damaged when Arcadion left it in Ultima VII Part Two. Repairing it with the Flux Analyzer, the Avatar used it to trap the Banes of Chaos.