I have a knife from Pesky right now where the darkening is definitely due to hand grubbiness. If you've ever seen how I work with my knives, you'd know that isn't going to be the source. The darkening is very consistent on the entire handle. For example, my DTITK (cocobolo) is now almost black unless you look at it under a nice, bright light, then you see all the reddish and hazel tones. I hadn't considered the UV but since my windows block almost all of it and never hit the knife area directly, I am fairly certain that is not the case.