E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Diamond Jousts (The).
Jousts instituted by King Arthur, who by that name had named them, since a diamond was the prize. Ere he was king, he came by accident to a glen in Lyonnesse, where two brothers had met in combat. Each was slain; but one had worn a
crown of diamonds, which Arthur picked up, and when he became king offered the nine diamonds as the prize of nine several jousts, one every year, a joust for one. Lancelot had won eight, and intended to present them all to the queen when all were won. When the knight laid them before the queen, Guinevere, in a fit of jealousy, flung them out of the palace window into the river which ran below. (Idylls of the King; Elaine.)