Golden Ammonite

This ammonite was in a piece of shale that I found at the base of a cliff in Whitby. It is only small, about an inch across, and was just a grey impression in the rock. It is a small garden 'ornament' and has been out the back for about a year.
Today with the sun on it I noticed it seemed to be glinting gold. I have no idea how this happened or what process has turned it from a simple impression in shale to what looks like gold leaf! Anyone out there got any ideas?