Have you ever seen a piece of obsidian? It's like black glass, sharp at the edge like a broken bottle. It is spewed out of volcanoes when they erupt, along with all the lava and other stuff. I've got some that I picked up in Iceland. There are so many volcanoes in Iceland that obsidian is all over the place. When you find a piece it seems quite special.
Arthur has a piece in this book. That is what his seeing stone is. Merlin gives it to him on his thirteenth birthday. At first Arthur doesn't know what to make of his gift, but later, as he handles his stone he begins to see images in the shiny black surface.
Now, who is Arthur? And, for that matter, who is Merlin? Arthur isn't King Arthur. He's Arthur de Caldicot. He's the second son and page of Sir John de Caldicot and he lives in the manor house at Caldicot. In the year 1199 he is waiting to hear what his father's plans are for his future. Is he to be betrothed to his cousin Grace of Gortanore? Is he to be a squire to his uncle Sir William de Gortanore, the necessary first step to becoming a knight? Or is he to be a priest and bookman? While Arthur waits he must practise his duties obediently.
Arthur keeps a diary. In it he will tell you all about his family and the people who live and serve on the manor. I wonder whether you would relish his life? It sounds hard and uncomfortable to me and burdened with superstition.