Good question - I've just read site description & watched the video which don't say. I assume the paper holders are plastic. They are 1 mm thick all over, by my ruler, and have a gently pebbly, not slick, texture. The site photo & video accurately portray the degree of translucency, and that they are flexible yet also firm (not limp), sturdy, lightweight - they will stand alone better the more paper they contain, particularly if you use them the tall way as the weight of paper will better flatten out the bottom where the assembly flaps are. I've recently purchased my second 3-pack of these for all my 12 x 12 designer paper pads & collection kits as well as loose sheets of patterns and solids. I consider myself a relative beginner at paper crafting bug, but know I'll buy more of these as my stash grows.