l’Isle d’Or, Ille Tornoiant, d’Isle Torneant, Isle Tournoiant, Isle of Turnance, Turnaunce, Turning Isle

Malory seems to name this site only once, as an island where the first Nascien spent eight days, some forty years after Christ’s Passion. It was composed of the waste left over when God separated the four elements. It contained a deposit of iron. The whole mass settled over a lodestone at the bottom of the ocean and the magnetic force caused it to turn perpetually. After spending some time on the island, Nascien left on the Ship of Solomon.

During Arthur’s reign, Merlin imprisoned the daughter of Duke Abinors, an enemy of Uther, on the island. She remained there, guarded by her lover Formis of Arms, until Arthur and Gawain set her free. Merlin’s Tower, which was once inhabited by the wizard, was situated on the island.

Surmising, however, that it may be the same island where Percivale spent some time during the Grail Adventures. If so, it is a likely place to experience visions and temptations when one is questing for the Grail. Turnance figures more extensively in the Vulgate account.

I would suggest identifying it with Lundy Island in the Bristol Channel.

See also
Marcoisa | The Legend of King Arthur
Port of Perilous Rock | The Legend of King Arthur