Unfortunately someone else's Magic stone cannot be used to upgrade your Minfernal. The stone is account-bound and you can't upgrade someone else's pet without trading the pet first. Minfernal is wild, so it can't be traded.
Try looking for a generic Flawless Battle-Stone instead, or you can trade 3 of the same family type stone to the Pet Battle Trainer in Stormwind or Orgrimmar for 1 of another type of stone.
Feel free to browse through Perks N Peeves, my vanity pet collecting blog!
Quintessence wrote:". . .or you can trade 3 of the same family type stone to the Pet Battle Trainer in Stormwind or Orgrimmar for 1 of another type of stone."
Oh so that's how that stone trade thing works. I never tried it, and I've been very fortunate with getting stones to drop through pet battles and dailies. Though I do still have about 15 pets on my must upgrade list.
Drrum, can you be more specific? I can go days doing the pandaria tamers and not see a stone drop. Are you saying you get on average a stone drop for every 2-3 tamers you fight? I am really scratching my head on this one, as I should be swimming in stones right now...