Nicknames

Hi! Can anyone tell me if the name "Petra" is a nickname for something else? I just received a treasure box of genealogy research from a relative who has passed. I have found baptismal and marriage information for Petra's siblings and parents but I would not have known about her, had I not inherited these notes. My next approach is maybe this is a nickname.The females, other than my great grandmother, are: Narcisa, Paz, Victoriana, Concepcion and Juana.
Thank you for your help.