Does a Knight of Ren Have VADER'S LIGHTSABER?

In the original concept art from Star Wars Episode 7 The Force Awakens, we get to see some images of The Knights of Ren. It's here that on one of them, we see what looks to be the lightsaber of Vader himself. Do you think this is Vader's lightsaber? Do you think Luke's crystal that he hangs from his neck is from Vader's lightsaber? Or is it from another Sith?

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