Matoya's Crystal Eye was stolen by Astos, and is retrieved after you defeat him in the Western Keep. Return it to Matoya to earn the Jolt Tonic which in turn lets you get the Mystic Key.
Matoya's CRYSTAL was stolen by ASTOS, and is retrieved after you defeat him in the Northwest Castle. Return it to Matoya to earn the HERB which in turn lets you get the mystic KEY.