They're made with Cascade Fixation Spray-Dyed Effects. The elasticity of the yarn makes for a very snug fabric with a cushy feel. I like ankle socks, so I stopped these pretty soon after I finished the heel turn.
I was surprised at how easy they were to make. I ordered Knitpick's 47" circs and used their free Toe Up, Two at a Time, Magic Loop Socks pattern. I'll be casting on for another pair soon!