Caddellite is a blue metal with very interesting properties. It can filter out harmful magical noises, as long as it covers the ears (simply holding the ears shut wouldn't do it). However, it is not a native metal to Britannia and can only be found in meteors.
In Ultima VII, the Avatar needed Caddellite Helms in order to breach the security measures of the Cube Generator. After consulting first the Time Lord and then Brion, the Avatar finally found a Caddellite meteor on the isle of Ambrosia, guarded by the Hydra Brothers. With the chunks of metal, it was easy for the blacksmith Zorn in Minoc to create the helms.
The name Caddelite derives from Michelle Caddell, Richard Garriott's personal assistant.