Magic Stone of Signet, also known as Magic Sig Stone, is a one-of-a-kind Repeat Item that appears in The Legend of Dragoon.
When using the Magic Stone of Signet, it has the same effect as the status ailment stun, but unlike stun it remains active even if the monster is attacked. The monster hit with the stone is unable to attack for three of its turns. Like the other unique items, it can be used once per battle and is not consumed when used. It can be sold, after which is it impossible to recover.
The Magic Stone of Signet is located in the Marshlands northeast of Hoax. When you enter the Marshlands head left, onto the large tree root, following it all the way to the next screen. On the next screen jump in the near-by boat and continue forward onto the next left screen. On that screen climb out of the boat onto the large octopus like tree and check the northern most chest and inside will be the Signet Stone.