O-Rings are not restricted to plastic or rubber. Any circular gasketing device can be considered an o-ring. In RADAR systems, o-rings are used to keep the pressurized non-arcing gas (usually dry nitrogen) in the waveguides. They are usually made of silicon because rubber can get heated and damaged when used in waveguides. Metal o-rings are used on high-stress, high-heat items, such as the solid booster rockets on the Space Shuttle.