sometimes when you roll new affixes on Pets, they will be max, but not all of the time. it would take forever to get all affixes on Pets to be max with Diamonds, since it rerolls all of the affixes, and who know if you will be lucky and have all of them Max with one roll? better to use Fluorite instead. if you run out of either of those, the only other way to improve your affixes on Pets is defeating Enslavers and finishing the Find Pet Feat, which improves the affixes on Pets. @Mr_Scooty has a cool Pet Farming Build that can speed up this process, if you want to go that way.
Crafting Pet Affixes 2: Calcite, Angelite, Quartz, Diamond, Larimar, Kyanite, Sapphire, Fluorite, & Topaz are the Crystals that can be used on Pets. Calcite is the only one that can be used on Eternal Pets. it costs 5 Crystals per use. Diamonds are better used for Legend & Crystal Affixes on your other items, unless you have a few thousand of them. at 5 Crystals per use, I went through 3k Larimar just to get Stealth on my Pet. think about it, if it takes you 30 attempts with a Diamond to get all affixes maxed, that is 150 Diamonds. but Diamond rerolls all affixes for random values, so if you got 4 maxed on a Legend Pet, and one isn’t, are you going to take the chance with Diamond to reroll all of them with Diamond one more time to get all 5 maxed? better with Fluorite one at a time, and smack Enslavers for Pets when you run out. this method really sucks for Pets crafted for PVP, but eventually they will max out, especially with Pets for PVE.