I'm thinking that it's a 'rubberized' plastic. Feels soft and smooth. I have a white/silver one (on sale at Borders in NYC just before they closed up) not my first choice but I figured that I don't actually look at the back very often and anyways no choice for a great price
There is some colour transference from my 'Roots' leather sleeve though. Brown on silver. When it gets too annoying I'm planning on getting a skin to make it new again.