the only Crystals that can be used on a Pet are: Calcite, Angelite, Quartz (don’t use this on a Legend Pet, you might regret it), Diamond, Larimar, Kyanite, Sapphire, Fluorite, & Topaz. it costs Five Crystals to change anything on a Pet. so if it takes you 5 tries to change your Pets Element from Poison to Fire, it would have cost you 25 Calcite (5 Crystals x 5 attempts to change the Element).
Eternal Pets have the same restrictions as other Eternal Items. so the only Crystal in the list that can be used on an Eternal Pet is Calcite.