Petra is such a nice name!!
Afraid I can only think of girl names 
Sabine
Cora
Lilith
Thea
Suvi
Sunniva
Dove
Nadine
Gaia
Cleo
Sylvie
Anya
Selah
Alma
Salome
Thora
Ines
Rose
Vera
Tamar
Sybil
Ingrid
Ione
Oona
Elke
Ianthe
Tamsin
Daphne
Cosima
Jorja
Annika
Ondine
Seren
Tirzah
Nadia
Elsa
Thalia
Juno
Selma
Maren
Linnea