Historical Fiction/Fantasy - Man imprisoned forced to create philosophers stone using alchemy notes. Spoilers ahead.

[Darrin]

I read this book around 2004 to 2007. It was a historical fiction/fantasy novel focused on creating the philosophers stone. There were two main characters, one man who was imprisoned and dedicated to the creation of the stone. I think someone had successfully done it before and this man was a witness to it. The other character was a man who thought it was all nonsense. He saw a trick where someone claimed to turn something into gold but merely coated it in gold. They may have been friends of some kind. The book did not appear old. It was a paperback novel written in English.

Spoiler below!
(view spoiler)[ The ending of the novel is what I recall most vividly. The stone had been created by the imprisoned man, and he gave it to the one who didn't believe it was the philosphers stone. He claimed it was a trick like he has seen before. The creator died while the second character tossed the stone into the sea or threw it out. He also took the item turned to gold and got on a boat somewhere. He began to slice back the layers of the object, sure he was going to be able to peel back the gold layer. Instead, he found the item was made completely of gold. He realizes the stone was real, and he tossed it away in his foolishness. (hide spoiler)]