Fourthwing theory I think that Violet is a pure source (we’ve heard this and know that she is comfortable wielding it as lightning) & that when Jack and her were sparring he was so enveloped in her power that he pulled from her & turned venin. We know using signets isn’t allowed on the mat and we also know that after turning venin you have the thin red lines around your eyes & then they go away as an initiate. Anywayssss, that’s my two cents!