Medieval fantasy book about a squire

Read most of this book years ago, in about 2002 or 2003. It was about two young boys who were raised by one of their relatives (I think he was one kid's grandfather). I remember a plot point early on was one of the boys had a chunk of amber and he broke a piece off and gave it to the other boy because he very obviously wanted it. There was a timeskip at some point, after which (I think) they were squires to a knight. The only antagonist I really remember was a merchant or something like that. At one point they confront him and he gets away, but the book makes it a point that they gave him a facial scar so they could recognize him no matter what disguises he tried or anything like that.