Just in case someone has missed it: You can buy Marked Flawless Battle-Stones. This type of Battle-Stone will upgrade a pet of any type to rare quality. You can buy it at the pet battle trainer and the cost to buy it is 3 flawless stones of the same type.
I tend to get stones I don’t really need so I think these are great. As soon as I have 3 of a type (that I don’t need) I go and change them over to a marked one.