I'll admit, I haven't been in this plane for very long, so I don't know a whole lot about Celebi or other legendary Pokemon or anything, but...

[ She grabs a green marker and heads to the board where she draws a little picture of a leaf and circles it. ]

Aren't there various stones that different kinds of Pokemon need in order to evolve? And one of them is a leaf stone, right? Could that be the "leaf of stone" that the riddle mentions bringing as an offering?