[name_f]Petra[/name_f] means rock/sone, so here’s some ideas to match that that also fit your ideas:
[name_f][/name_f][name_f]Harlow[/name_f]
[name_f][/name_f][name_f]Lana[/name_f]
[name_f][/name_f][name_m]Callan[/name_m]
[name_f]Girl[/name_f] Names that Match:
[name_f][/name_f][name_f]Lola[/name_f]
[name_f][/name_f][name_f]Sara[/name_f] or [name_f]Sarah[/name_f]
[name_f][/name_f][name_f]Sage[/name_f] or [name_f]Saige[/name_f]
[name_f][/name_f][name_m]Jess[/name_m]
[name_f][/name_f][name_f]Willa[/name_f]
[name_f][/name_f][name_m]Morgan[/name_m]
[name_f][/name_f][name_f]Samantha[/name_f] (nickname [name_m]Sammy[/name_m] or Sami)
[name_m]Boy[/name_m] names I think would go well:
[name_f][/name_f][name_m]Jordan[/name_m] (gender neutral + [name_f]Petra[/name_f] is the capital of [name_m]Jordan[/name_m] so kind of fun connection)
[name_f][/name_f][name_m]Luke[/name_m]
[name_f][/name_f][name_m]Zane[/name_m]
[name_f][/name_f][name_m]Eli[/name_m]