Battle-Stones are, to me, one of the best and most aggravation saving items added to the Pet Battle system in the 5.1 patch, and are second only to the bandages in usefulness in the overall mini game. There are two levels of Battle-Stone available, in green and blue. For 1,000 justice points you can buy a Polished Battle-Stone that will upgrade any pet up to uncommon quality at the cost of up to two of it’s levels. I do not think I am exaggerating to say that I doubt these have seen any significant amount of pick up unless you desperately want to see a pet as a better quality, but have seen no flawless stones drop.
Second, and the most desirable and popular stones, are the Flawless Battle-Stones. These are obtained via wild pet battles and the pet battles daily quests that reward the pet bags. It is rare to find a Flawless Battle-Stone from these sources, which will upgrade a pet to rare quality. These come in the general stone, which can be used on any pet, and the family specific ones that can only be used on pets in the specific family, and all will cost up to two levels of the pet.
With the rarity of these stones, sometimes less than 1 in 50 battles or worse, it can be very hard to choose what pets to use the stone on. My general rule is that if the pet has a lot of campers, and a limited spawn, it could be a good target for the stone. Also, if it is a pet you learn from an item, like my beloved Ghost Skull named Zarhym, then that would also be something you want to use a Battle-Stone on. In general, you want to save your first few stones for pets with unique abilities for their family. An elemental pet with beast attacks? Perfect! Use them to improve the pet you have with the best attacks, but which isn’t yet rare, and that you would have the hardest trying to farm for a rare quality.
Let me explain the pets I have used stones on. The pets I have used Battle-Stones on are: Wanderer’s Festival Hatchling, Minfernal, Ghostly Skull, and Baby Blizzard Bear. The reasons I choose these are pretty obvious if you know nearly anything about these pets. The Wanderer’s Festival Hatchling and Minfernal are two pets that are extremely hard to get. While I did get them, the chance of finding a rare, and odds of going back to try over and over for a rare quality one are pretty far off, so I just settled for what I could get. The Ghostly Skull and Baby Blizzard Bear pets are both learned from items, so there is no ability to farm those pets to get a rare one.
Therefore, my best advice to you is to make sure that as many of your pets as possible are found as rares. However, those that you know are hard to find, or which are learned from items, should be the ones that you save your stones for when you have them. If all of your pets that you would normally would spend a stone to upgrade are already upgraded, then feel free to use your extra stones on pets you could, but have had no luck farming up a rare of. The next time I have a flawless beast family stone, my Arctic Fox Kit will be getting an upgrade because the one I got took me over a week of camping in a place that looks far too much like my hometown in January.
One thing I want to note here is that Blizzard has files in the game for Battle-Stones that would upgrade a pet to epic, however there has so far been no indication of when or if those stones will be made an item you can get in game. In the meantime, the only folks with epic pets will remain some of the higher level trainers. I doubt we will ever see the ability to make a pet Legendary like some of the top level trainers in Pandaria have. Are there some pets you are looking for that you have a stone in mind for, or are you all levelled up on the pets you had planned to upgrade? Let us know in the comments!