A short story Niy did, which was included in the Wakasagihime fanbook called "サヨナラ、王子サマ" (Sayonara, Ouji-sama). Niy's coloured works are beautiful. This was translated by sunember123 on danbooru. Thank you very much. If you were wondering what that little reference to collecting stones was in Sekibanki & Youkai Mountain, here's your answer.
Translations from danbooru: https://danbooru.donmai.us/pools/10618 Raws from pixiv: https://www.pixiv.net/en/artworks/53490582