Weapons made of Escudo were a Requirement for Hikaru, Umi and Fuu to become Magic Knights as the evolving Weapons were the Key to awakening the Rune Gods. Escudo can only be found in the Legendary Spring of Eterna. According to Presea numerous Warriors have journeyed to Eterna in hopes of finding Escudo but none have ever returned.
Presea also mentions Minerals such as Arl, Slye and Kusto which are apparently strong but not good enough for Magic Knights.
Escudo is named after the Suzuki Escudo.