Disclaimer: I'm not muscling in on JK's turf - just gambolling on it, like a spring lamb, having fun working out the literary and psychological puzzles which she is having fun setting us.
"It's only possible because we have his somatic memory at the point of death. Gwydion made Blodeuwedd's body entirely from flowers but it's a bit more... metaphorical for a man."
"Blodeuwedd was the one who killed her husband?"
"And was Transfigured into an owl, yes. The best post owls descend from her." Raising her wand, she began the incantation.
"I have transfigured you a body out of stones and flowers... what?"
Ron gave her a wall-eyed look. "Isn't that a bit - well - nancy for Snape?"
"It didn't say which stones and flowers, so I used flint and deadly nightshade."
"Granite and deadly nightshade" would actually have had a better ring to it, but I though flint was the more appropriate stone - since it is not only hard but sharp, and it strikes fire.
Gwydion is a famous wizard who appears in the body of early Mediaeval Welsh tales called the Mabinogion. He makes a woman called Blodeuwedd out of flowers, so that she may marry his nephew Lleu, who has been cursed to marry no human woman. Blodeuwedd however falls in love with someone else, and she and her lover trick Lleu and kill him. Being magic himself he doesn't die but changes into a wounded eagle which is later restored to human form, and Gwydion punishes Blodeuwedd by turning her into an owl.
"Stones" is of course old-fashioned, rural slang for testicles.
This story has now been expanded into a multi-part fic under the name Giving Extras.