I believe this is from the mid-80s. The plot centers on a nobleman who is VERY far down the line from the king suddenly being presented with the crown. He does not care for governing and there is a challenger to the throne. He discovers that the crown has magical powers since it is made from “sunstone” (not sure if that’s the term in the book, but it’s the basic idea).
The pretender is holed up in the wilds somewhere near a lake. Turns out he is collecting small bits of the stone to make his own composite crown. I remember at some point the protagonist being caught by the pretender’s people when his crown got either wet or covered in snow, losing its powers. Not sure how he escapes, but in the end he uses this information to defeat the pretender by getting the pretender’s crown wet or cold. I think one incident involved cold water and the other snow.
Other things I remember: 1) don’t recall the writer, but I think they had another critical success in the 90s. I wanted to say Connie Willis’ To Say Nothing Of the Dog, but she doesn’t have any book series like this. 2) As I said, I think it is a series, though I only read the first book. 3) Cover was one of those typical 80s things—had a frame with the title, etc. then a painting of the characters. Similar to the Deryni books from the 80s (added below), but I feel the painting was a bit more cartoony.