Sangraal
THE STARGATE OMNIPEDIA
SANGRAAL
A sacred stone the color of blood, believed to contain great powers. It has also been called the Bloodstone, and the Holy Grail. Merlin wore it around his neck.

King Arthur and his knights departed Camelot in search of the Sangraal, and quested it in three locations: Castiana, Sahal and Vagonbrei.

The Sangraal may be the source of Merlin's anti-Ori weapon.

KEY EPISODE(S) -
Camelot - Daniel deduces the item SG-1 is searching for is likely the Sangraal. Unfortuntaely Arthur left Camelot in search of it and has never returned.
The Quest, Part 1 - SG-1 discovers the planet where Merlin likely hid the Sangraal, but must get past a series of obstacles set by Morgan Le Fay before they can reach it.