The sword in the stone is usually supposed to be related to the Volsung saga in Norse. In that saga, Odin drives a magic sword into a tree as a gift for the man who can remove it. Only Sigmund is able to free it. This sword later breaks, and its shards are re-forged by his son (with assistance from a magic dwarf). The resulting sword, Gram, is the sword Sigurd uses to kill the dragon. This sword is the clear predecessor for Tolkien's Narsil/Anduril, the sword that was broken. The story of Gram is not completely unlike the Arthurian story, with a divinely-given sword, the freeing of which results in a test, and which later breaks and must be replaced. In Malory, the sword in the stone breaks in a fight with king Pellinore, from which Arthur is only rescued by Merlin's intervention. Merlin then takes Arthur and introduces him to the Lady of the Lake, who grants him Excalibur. However, in earlier versions of the story, Excalibur and the sword in the stone are the same sword.
I remind you of all of this to pass on another possible origin for the sword in the stone story, via D29. The timing is just about right. It will have happened some decades before Robert de Boron's poetry.
The legendary sword in the stone still stands in Italy. While connected to Arthurian legend and British history, this Sword in the Stone is associated with a Catholic saint. Visitors can see it in the Montesiepi chapel, near Saint Galgano Abbey in Chiusdino, in Tuscany.A video of the sword and the stone, around which was later built an abbey, can be seen here.
The legend surrounds the story of brave knight Galgano Guidotti, who was born in 1148 near Chiusdino. After spending his youth as a brave knight, Galgano decided to follow the words of Jesus in 1180 and retired as a hermit near his hometown.
Galgano is said to have stuck his sword onto a rock in order to use it as a cross for his prayers. One year later Galgano died, and in 1185 Pope Lucius the 3rd declared him a saint.
After Galgano's death, according to legend, countless people have tried to steal the sword. In the chapel you can see what are said to be the mummified hands of a thief that tried to remove the sword and was then suddenly slaughtered by wild wolves.
The sword was believed to be a fake for years. However, recent studies examined the sword and the hands, and the dating results as well as metal and style of the sword all are consistent with the late 1100s, early 1200s. This lends credence that the story on which the English sword and the stone is based on originated with Guidotti in Italy.