I've seen stone flowers and plants adorning headstones, but this is the first passion flower I've seen.
The passion flower has been used as a symbol of the passion of Christ since the 16th century. Spanish conquistadors, perhaps in an effort to convert South American indigenous peoples to Catholicism, named the flower and used its parts to tell the story of the death of Christ. The petals and sepals represent the ten faithful apostles, the five anthers represent the five wounds, and the three stigmas represent the three nails. The flower also has a corona of thin lobes (although not carved here) that represent the crown of thorns.
The passion flower vine climbs up the cross, the ultimate symbol of the passion.