Caliburn

Caliburn is the holy sword that symbolizes England's king of knights, king Arthur, which he pulled from the stone of selection under the guidance of the magus Merlin. Saber has stopped aging since then. It is the golden sword Emiya Shirou traces in the Fate route of Fate/stay nightwhen driven into a corner.

Saber seems closer to this holy sword, but it is already lost forever. In the legend, it is said that king Arthur broke it in a battle where he strayed away from the code of chivalry.

It looks like Excalibur, but it is a symbol of the king's authority; as an ornament, it has lost its effectiveness as a weapon compared to Excalibur, in exchange of that beauty.

Although they look similar, Caliburn is a different sword from that granted by the fairy of the lake, and it has different characteristics as a Noble Phantasm. Still, Shirou feels they are still in the same category.

When Shirou traced it, it apparently had stored prana, and with the help of Saber, he could kill Berserker seven times over in one strike. Of course, it couldn't compare to Excalibur as a Noble Phantasm, but it is the "strongest holy sword" it is being compared to, afterall.

Its rank is unknown, but the traced version was able to hit Berserker: counting his God Hand and the fact that a weapon traced by Shirou looses a rank, it is assumed that the original has to be at least ranked A.

Sources: the Tsukihime Laboratory entry and the weapon status screen, translated by Mirror Moon.