The baby volleyball is a pearl that is mistaken for a volleyball by Patrick. It appears in the episode "Porous Pockets."
The pearl is light purple and is very shiny and round. It is extremely valuable.
Role in episode
SpongeBob thinks it is a baby, but Patrick thinks it is a volleyball, so SpongeBob comes up with the name of "baby volleyball." They start playing volleyball with it until the appraisal worker tells them it is of great worth.