The Stone Knife was a famous object, best known for being used by Queen Jadis to sacrifice Aslan on the Stone Table. As its name suggests, the knife had a blade that was made of stone, and was noted to have "a strange and evil" shape.
After Jadis' defeat, the knife was somehow taken away and placed on Aslan's Table on Ramandu's Island. By then it had become a holy item: When three of the Seven Lost Lords arrived on the island, one of them dared to touch the knife in a fit of anger, which put both he and his companions into a deep slumber.
The knife most likely remained on Ramandu's Island until the destruction of Narnia.