Can't say for sure, but I'm guessing plastic. I recently picked up a Moto X 2014 with the bamboo back and love the look, but it is rather slippery. My prior phone was a Moto X 2013 and I had the woven black back which had a soft touch feel and wasn't as slippery. Not sure about the 2014 models, but with the 2013 ones, generally the darker color plastic backs had the soft touch grippier backs while the lighter colors were more slippery.