I read both that it has to be in the 2nd slot or in the 6th slot or even battling a wild Growlithe xD Was it in the slot next to Pikachu by any chance?
It is weird. Apparently there are other Pokemon with the same codes as Leaf Stone, Water Stone etc. as well. I wanna test this out now!
* Fire Stone = HEX 20 = Special 32 = MissingNo. (no other MissingNo will work, only the HEX 20 version)
* Thunder Stone = HEX 21 = Special 33 = Growlithe
* Water Stone = HEX 22 = Special 34 = Onix
* Leaf Stone = HEX 2F = Special 47 = Psyduck
* Moon Stone = HEX 0A = Special 10 = Exeggutor