Neoprene is my guess, the standard 2" collars. Yea they fit in the lid of the cloner, it looks like an injection mold of a 5 gal lid top with eight 2" depressions in it for the collars. I really like it, i didn't plan on any of the clones to take. I cut them off the plant, just straight not at a 45 or anything, dipped it in some clonex popped a collar on and then into the lid it went. All four took me less than one minute to stick. I plugged it in and never turned it off. Check water temp after a few days 74F took the cuts Dec 1st, they threw roots on the 11th....how much easier can that get, and these clones were taken 9 days into flower when i did my last cut back....my intention was to test the cloner under horrible circumstances and see how it held up / temps/ all that jazz for a few days, but the cuts just keep growing in the cloner and they looked so good I never shut it off....I'm sold man I love this thing.