(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).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894