What is a "chrysknife" in the novel, Dune?

The Fremen's basic weapon, the chrysknife is ground from a sandworm's tooth and provides free passage to any sietch for any blue-eyed man. The Harkonnens put a million solaris reward for anyone turning one in. The Shadout Mapes gives one to Lady Jessica once she determines Jessica is the One prophesied in the ancient legends and Duncan Idaho claims the one belonging to Stilgar's old friend Turok, whom Idaho tries in vain to rescue after a Harkonnen assault.