Petrone Vermiglio
A holy site on the island of Matufer off the coast of Cornwall.
Anyone who touched the stone could only tell the truth. Mark brought Isolde there to discover the truth about her affair with Tristan. Tristan, disguised as a pilgrim and a madman, grabbed Isolde and kissed her in front of everyone. Then, Isolde was able to swear that no one had used her body basely except Mark, the “pilgrim,” and the “madman.”
Source
La Tavola Ritonda | 1325–1350