Izkael said: isn't that kyouko's chest ornament not a grief seed?
No, that is indeed a grief seed. The 'C' mark at the top of Sayaka's soul gem is identical to the one on the grief seed. If you watch the scene when this happens, this is what the grief seed looks like as it is forming from a soul gem. I also see that the light angle does produce a shadow that is in the shape of Kyouko's chest ornament. That makes me wonder if the artist is symbolizing Kyouko's sacrifice as well in a less obvious manner, or its appearance is mere coincidence.