A regular type of Mystery Disc made of a silver alloy. It can be seen as an item in Monster Farm Online and Monster Farm Lagoon that allows you to unlock special monster breeds, and is part of a story plot in Monster Rancher Battle Card. One sits in the Aurora city shrine and is stolen by one of the kids trying to get attention. Johanos, the city elder, asks you to retrieve it for him.
Besides just the difference in monetary value, Silver Discs are thought to be precious because they were usually commissioned by powerful and wealthy people in ancient times who had access to more powerful and rare breeds of monsters. Thus those who find them hope that the monsters contained within them will be extra special.