They are more ordinary rubber (wash well, they feel still strong) and metal. The rubber insert part has several diameters so unlike some reviews, this is adaptable: it is not required to push all that much gets it does its job.
It goes on all my liquor bottles make for yourself (that said, for those who want to put that on their bottle of oil I understand not really because the rubber will melt on contact with the long-term oil) .. .