Crystal Isle, Isle de Voirre
An otherworldly island, ruled in Welsh legend by King Maelwys (Melwas), who abducted Guinevere. Chrétien de Troyes names its ruler as Baron Moloas, which is probably a variation of Maelwys.
According to Chrétien, the Island of Glass never had storms, was devoid of toads and snakes, and always had a perfect temperature. This description evokes an image similar to the Isle of Avalon, and it therefore supports the identification of Avalon with Glastonbury, which some writers though was once called the Island of Glass.
In the Vulgate Lancelot, the Island of Glass is the home of Sir Mador the Black. Hartmann von Aue calls this realm the Crystal Isle.
Sources
”Dialogue of Arthur and Gwenhwyfar” | 16th century
Erec | Chrétien de Troyes, late 12th century
Vulgate Lancelot | 1215-1230