Abstract: An improved ball valve for lethal gas or fluid service is disclosed having a novel triple redundant stem to bonnet seal design. A ceramic seal ring forms the primary stem to bonnet seal. The ceramic seal ring has a first seal surface lapped for sealing engagement with a corresponding lapped surface on the stem and a second seal surface lapped for sealing engagement with a corresponding lapped surface on the bonnet. The ceramic seal ring first seal surface lapped for sealing engagement with a corresponding lapped surface on said stem and the corresponding lapped surface on the stem may be frustoconically or spherically shaped. An elastomeric seal and a compression type seal between the stem and bonnet act as redundant seals. A preloading means to maintain the ceramic seal ring and lapped sealing surfaces on the bonnet and stem in sealing engagement is shown.