The Dagger of Lex was one of the many artifacts available for purchase at The Magic Box. Buffy Summers' only use of it was to stab a mummy hand to obtain it for a customer.
Behind the Scenes
The Mother of Pearl Handle on The Dagger of Lex was created using pearlescent nail polish.
The same knife that was used for "The Dagger of Lex" was also used as Andrews Knife in the Seventh Season Episode: "Storyteller", though it had runes added to the blade and the Mother of Pearl removed from the handle.