Disc Stones or Saucer Stones are flat spherical stones that contain a monster's genetic information inside. You can have them appraised at a Shrine, but there is no way of knowing what type of monster is inside until it's unlocked. For this reason they are sometimes referred to as "Mystery Discs".
In most Monster Rancher videogames, these stones are represented by the CD's the player uses. Each CD contains information that helps you unlock a specific monster breed.