They're made of balsa wood like you get from craft shops.
Some firms varnish tme, though - there was a debate about whether this could harm the fish a few years ago, which was went most people switched to poppers of one kind or another.
You can either pay a small fortune for these or make your own from corks, polyballs etc.
There's a thing in Coarse Fisherman - or a plug for Fox, depending how you look at it - showing how to rig them up this month.