The Seeing Stone: Kevin Crossley-Holland

Book I of Arthur. Thirteen-year-old Arthur de Caldicot lives on a manor, desperately waiting for the moment he can become a knight. One day his father's friend Merlin gives him a shining black stone - a seeing stone - that shows him visions of his namesake, King Arthur. The legendary dragons, battles, and swordplay that young Arthur witnesses seem a world away from his own life. And yet there is something definitely joining the Arthurs together.
It will be Arthur de Caldicot's destiny to discover how his path is intertwined with a king's . . . for the past is not the only thing the seeing stone can see.