Board Thread:Theories and Speculation/@comment-12654584-20180815152132/@comment-12654584-20180815193035

No, the Yasaka Magatama is not it. The individual projectiles are shaped like the Yasakani, but the Yasakani's shape isn't exactly unique or uncommon in Naruto (tomoe are everywhere in this series), and the Yasaka Magatama attack is produced from the Susanoo's body/miasma, whereas the tomoe-shaped thing in Shisui's Susanoo doesn't seem to be (as it is a totally different color than that of his Susanoo and he never has his Susanoo use it as a weapon or separates it from his Susanoo's body). Also, the Yata Mirror, Kusanagi, and Totsuka sword are also all specifically talked about in the series as super-duper-special-and-unique, one-of-a-kind objects and weapons, whereas the Yasaka Magatama is described as a technique derived from the the Susanoo, an "any sufficiently developed Susanoo (or at least many sufficiently-developed Susanoo) can do this" kind of thing.

Saying the Yasaka Magatama is the Yasakani jewel is nonsensical and basically equivalent to saying that it simply doesn't exist in Naruto (and perhaps was never planned to), which is the answer I'd find more logical.