Mushroom Knobs look similar to mushrooms and can be used to perform an assortment of functions. Reid Supply offers the Phenolic Mushroom Knob and the Plastic Threaded Hole mount type and the Threaded Insert mount type. Available are different knob diameters ranging from 1 -1 /4 inches to 2 – 13/64 inches. These Mushroom Knobs’ stud material is commonly made of plastic or brass.