Talk:Characters/@comment-175.143.20.205-20150531142127/@comment-26093926-20150531144739

Pretty much whatever the game devs decide the rarity. Lot of it is historical trivia and which swords are famous. For example, the 4-petal rarity swords are imperial treasures (I think Hasebe and the 30 min tantous are also imperial treasures which is why they have longer timers). Mikazuki is a 5-petal rarity because he's a member of the Tenka Goken (Japan's 5 famous swords). Other swords aren't regarded as rare because they are speculated to be mythical (ie. Iwatooshi and Kogitsunemaru)