I'm having trouble remembering the title of a book a read when I was in middle school but I'm compelled to track it down again years later. Here are the details I remember : The main character was a young girl. She comes to stay with this Lord who is her senior by several years and they start slowly falling for each other. They realize they are meant for each other in a scene where they jump over a bonfire on midsummer. They break peat bricks open and the filaments match the hair of the other. The tradition was that the color of the filaments in the peat brick would be the hair color of their soulmate. The hair colors were flaming red and bright silver. I believe she is taken at one point and imprisoned in some catacombs as well. The Lord was a sailor too, which was a focal point of the story. And they made quite a deal about the castle he presided over and mentioned many out lying building that I can't quite seem to remember.
Those details stick out to me clearly but beyond that I can't remember anything and it is unreasonably frustrating. Please help if you can. And thank you for your consideration in advance.