It's just a hair ornament. Wiki calls it a barrette. She's said in one of the end-of-volume Q&A sessions (the second-to-last page in these volume extras .5 chapters) that it's made of deerskin. Tons of backstory details also get revealed in these Q&A sessions, like the one for this particular extra chapter that say she drifted across the Pacific Ocean alone, or the fact that she's visited Mexico when it was ruled by the Aztecs. (the question she answers 'Azteca' here)What's the thing behind her head