That would be fen (.01x). They are made of plastic as their value is so low. Mao (.10x) and Yuan (1x) are more like real coins, though mao's are smaller than yuan, and they have a plastic mao variant also, I believe.
Fen probably wasn't eliminated due to inflation fears. For the same reason, they won't issue a bill larger than 100RMB even though it would be very useful.