This is how Kylo Ren's lightsaber works from the Force Awakens Visual Dictionary

Yes, I bought the Visual Dictionary for the Force Awakens because I'm really nerdy and you all should know that. The freakin' illustrations are really cool, and they answer a lot of questions. But the big one that I had (and I know you probably have too) is why does Kylo Ren's lightsaber look so rough and jagged?

Well it turns out that he's using a cracked kyber crystal. It's also an ancient design that (according to the visual dictionary) "dates back thousands of years to the Great Scourge of Malachor. The crossguard blades, or quillions, are tributaries of the primary central blade."
Pretty cool, right? You can thank me in the comments. Oh and I will be posting more cool stuff from the visual dictionary. I love how detailed the Star Wars universe is. But I guess you can afford it when you are the movie with the highest gross money grab in history.