A sword embedded in a stone has special significance to stories of King Arthur. According to legend, Arthur once pulled a sword from a stone as proof of his righteousness and royalty. It is a common misconception that the sword was Excalibur, for it was only after the sword from the stone broke in battle that King Arthur was given Excalibur, a sword forged by the Lady of the Lake.
In the cavern beneath Glastonbury, once known as Avalon, SG-1 discovered a large stone pedestal, and upon their arrival, a holographic sword appeared in the stone. However, after it had materialized, the sword took on physical properties and could be handled as a solid object. Mitchell's first attempt to draw the sword failed, but an image of Merlin appeared, entreating them to prove their worthiness. After SG-1 had passed the tests to prove wealth of knowledge, Mitchell successfully withdrew the sword from the stone, causing a holographic knight to appear. Mitchell battled the knight with the broadsword, but the sword was designed to sense whomever had pulled it from the stone, and to the other team members the sword remained a hologram. Mitchell defeated the knight, and when he returned the sword to the stone, Merlin's treasure materialized and filled the chamber.
SG-1 also encountered a sword in a stone in the public square of Camelot on PX1-767. The young men of the village who were preparing to be knights would test themselves against the sword. They trained to be ready to serve King Arthur, whom they expected would return one day and would need knights to join him on his next crusade. According to legend, only he who is pure of heart and true of spirit can pull the sword from the stone, and it is said that Arthur will not return from his quest until one among the villagers succeeds in removing it. When the Black Knight terrorized the village, it was Valencia, a young woman, who rushed to Mitchell's aid and drew the sword from the stone, tossing it to him to battle the knight. Although the knight was deactivated from the control pedestal in Merlin's library rather than from Mitchell's sword, it was Valencia who was seen as a hero. Her bravery and her actions were considered a sign that Arthur would soon return.
Cross Reference: Avalon, Black Knight, Camelot, Glastonbury, Holographic Projection, Valencia
Episode Reference: Avalon, Camelot