Battle-Stone

Battle-Stones are items used to upgrade a battle pet to uncommon or rare quality.

 [Polished Battle-Stone] can be used to upgrade a pet to uncommon quality, while all kinds of Flawless Battle-Stone will upgrade a pet to rare quality. Polished Battle-Stone and the generic  [Flawless Battle-Stone] can be used on any pet, while all other kinds can only be used on a pet of the appropriate type.

Source

Flawless Battle-Stones of all kinds have a small chance to be obtained upon winning a battle against wild pets. Type-specific Battle-Stones also have a small chance to be obtained from the  [Sack of Pet Supplies] awarded from defeating master trainers.

Players can also trade in any 3 of the same family specific Flawless Battle-Stones for a single  [Marked Flawless Battle-Stone].

Battle-Stones

•  [Polished Battle-Stone]
•  [Flawless Battle-Stone]
•  [Flawless Aquatic Battle-Stone]
•  [Flawless Beast Battle-Stone]
•  [Flawless Critter Battle-Stone]
•  [Flawless Dragonkin Battle-Stone]
•  [Flawless Elemental Battle-Stone]
•  [Flawless Flying Battle-Stone]
•  [Flawless Humanoid Battle-Stone]
•  [Flawless Magic Battle-Stone]
•  [Flawless Mechanical Battle-Stone]
•  [Flawless Undead Battle-Stone]
•  [Marked Flawless Battle-Stone]

Patch changes

• Patch 5.4.0 (2013-09-10): Players are now able to trade in any 3 of the same family specific Flawless Battle-Stones for a single  [Marked Flawless Battle-Stone].
  
• Patch 5.2.0 (2013-03-05): Now have a chance to be awarded from PvP Pet Battles.
  
• Patch 5.1.0 (2012-11-27): Added.