Well... If we're going by MFC...
The Fraggles explain the order of whose hands the pebble has passed through. If I had my VCD player hooked up now I could go and watch and come back to post. Got my VCR/DVD combo hooked up instead for video cassettes though.
After Mokey gives it to Boober, he reaches up to give her a little peck on the cheek. Before the frogs leave, Boober gives the pebble to Robin. And at the end, during the caroll medley, Robin gives the pebble to Grover while the two re in front of the tree.
Course, in a fictionalized account, events can/may be changed to protect the innocent. If Bo changed it in his "Robin's Story" to have Grover give it back to Robin, that's OK within the bounds of his own story. After all, he's the author and he's the one shaping the events of the story. It doesn't hurt that it's within Fraggle nature to keep passing the pebble from person to person, so it'd be plausible (thank you Walt Disney for that word) that Grover could've given the pebble back to Robin. But that's neither here nor there, just thought you might like to know to clear up any confusion.