Merlin’s tomb, where Tristan and Lancelot fought a great battle in the Italian Quando Tristano e Lancielotto Combattettero al Petrone di Merlino.
Merlin had predicted that the two greatest knights in the world would do battle there. Tristan thought he was fighting Palamedes (Palomides), who had agreed to meet him there but was delayed by injury. Lancelot, for his part, expected to stop a fight between Tristan and his son Galahad. The two knights revealed themselves and ended the fight before they injured each other too severely. This episode appears, in varied forms, in many different romances.
In La Tavola Ritonda, a number of stones of this nature are collectively called Merlin’s Stones.
Source
Quando Tristano e Lancielotto Combattettero al Petrone di Merlino | Late 15th century