They are bubble wrap on the inside and a paper type of product on the outside.
I did the "puncture" test on one of them...because I wasn't sure either.
I took one of my unused envelopes and sacrificed it...lol. I poked my fingers at it and tried to stab it with the corner of another envelope to see if it would tear all the way thru or not. It only tore the surface after repeated tries...but the bubble side stayed intact. So I figured it would be okay. I still taped all the corners, the addresses, and the edges.
Time will tell.