There's a gray/silver residue on my iPhone 5 usb cable, what is it?

I noticed the other day that when my usb cable (the actual usb end, not the end that plugs into the iPhone) brushed across my skin it left a dark gray mark that could be easily brushed off, on closer examination it appeared to be shiny, almost like pencil lead. Because i have really bad OCD, I borrowed my sisters ipod cable (ipod 4, I'm not sure if that makes a difference or not) and brushed it on my skin, and nothing came off, I even plugged it into the usb drive that I usually use with my iphone and again nothing. The reason I ask this is because my iPhone appeared to be partially opened when I bought it and because of my OCD of course I go straight to the idea of someone tampering with it. I would just like to know if the gray substance is maybe scratched off paint or something or if could possibly be something dangerous. Note that the gray stuff can be rubbed fully off, but when I return it to my usb drive it is again covered by the gray stuff, but like I said it isn't anything inside my drive because nothing came off on my sisters usb.

Don't care