Tomoe

A tomoe or tomoye (archaic) (巴) is a Japanese abstract shape (i.e. a swirl) that resembles a comma or the usual form of magatama. It closely resembles the symbol of the beast (666 in a circle), which leads on from the idea that Sasuke's chakra is "cursed", and may have to do with the Sharingan's "true" purpose (alluded to by Tobi).

They appear in the Sharingan eyes, and they also form the Cursed Seal of Heaven which Orochimaru put on Sasuke Uchiha and Anko Mitarashi