It is known that Black Gold Saw has some sort of hidden agenda; in episode 4, when she is choosing a cup.It is possible that the black icons on the cup represent each student who lost their memories. The fact that when Kagari eyes turned into the "gear" like pattern.
The change in Kagari's eyes.Kagari acts cold towards MatoYomi's eyes however are already have the "gear", Mato too.
The eyes of Yomi.Oh no! Kyubbey has turned them into witches. XD. Refer to Mahou Shoujo Madoka.