A pin shaped like a crescent.
A regular feature of the outfit of Patchouli Knowledge, as well as all of the Mutsuki-class destroyer shipgirls in Kantai Collection:
* Mutsuki
* Kisaragi
* Yayoi
* Uzuki
* Satsuki
* Minazuki
* Fumizuki
* Nagatsuki
* Kikuzuki
* Mikazuki
* Mochizuki