pure cork. Simple in design, they're comprised from granulated cork pressed together and bound by an adhesive, and then turned on a lathe. They're relatively light, extremely durable, and of course, sustainable.
Turns out cork is pretty versatile. (See how cork is made here.) It's tough enough to be used as flooring, light enough to be used in space-shuttles, comfortable enough to be used in shoes, and naturally water resistant. Jasper Morrison designed these stools for Vitra and they're available at a number of only design centers including Design Within Reach.