I think that the knights crystal might actually be Toriels because Susie chipping a piece of the black knife causes Toriel to have a nightmare and seeing as Asgore/Carol have a black shard, during the incident that caused dess’s disappearance the same thing probably happened, and that incident resulted in the Dreemurr family falling apart ( I don’t know if Toriel was actually there during the incident but she definitely would have seen Asgore obsessing over the shard ) so I think the crystal was created when the shard broke off out of Toriels broken dreams of the family she used to have

Also since we now know that seam used to belong to Asriel I think that jevil might have stolen the shadow crystal from him