I think the original one was probably bakelite - polyoxybenzylmethylenglycolanhydride -apparently, then followed a series of corks that disintegrated, then a rather beautiful brass curtain rail finial which can be seen - if you really have nothing else to do - on my post-materialism site; then . . . err, ah yes, a lead penguin, then this (above) lead sheep, although without its modernistic wire construction - this added when the wire-wrapped-around-belly-design finally failed.
Unless the vessel itself melts through being boiled dry - something my mother excelled in, I think it will be with us for ever, the knob being changed every six months or so when the last one fails, or something else presents itself as being a super alternative.