There are several symbols in the novel, a major one being the birthmark over Sula's eye. The mark looks like a rose, but later in the novel, Nel says that the mark is getting darker, and Jude thinks it looks like a snake's head. The mark symbolizes the element of evil that the townspeople in Medallion think Sula has.
The birthmark on top of Sula's eyes look like a snake to some and to some it looks like a rose which all depend on perception and how they view Sula. Birds are also a huge aspect in Sula because birds are everywhere and a canary is used to describe someone, yet robins follow Sula and this is viewed as evil which showcases the superstition in the town.