A Koan is an item in the game Chu-Teng. Judging from gameplay, they are created whenever you subdue darkness. They cannot be kept in Rin's furoshiki, so in terms of gameplay, you can't actually use them.
The first time you see Koan in-game is in the Milky Way, when Takochuu drops his mines onto darkness amoebas, turning them into Koan. There are also two "Koan dumps" in the game; one in the Star Palace and another in the Sun Palace.