Made of Crystal Clear Vinyl. Metal Hook Will Not Rust.   CRL Vinyl Suction Cups contain U.V. stabilizers that protect the cup from yellowing in the sun. Cups generally adhere to smooth, non-pourus surfaces with little effort. Designed to hold signs, radar detectors, thermometers, bird feeders, and so on, this cup will carry up to three pounds (1.3 kilograms) of weight. Diameter: 3/4 in (19 mm). Important Note : 25 Pack