The Big Top Crime (Series)

A fox steal the world's most valuable diamond and buries it under a tree, believing it to be well-hidden. The next day, a circus arrives and sets up its tent next to the tree, its animals unwittingly digging up the diamond they believe to be just a worthless rock. They soon lose the diamond inside the circus and the desperate fox attempts to retrieve the stolen diamond before the other animals realise what they have. (Source: AniDB)