Did a lot of research yesterday on the internets.
I'm 100% sure it is a jeweler's fake.
I even popped it out of the setting and the ridges
are only on the top.
Then I found a forum
with a post about the exact same coin
with ridges missing in the exact same place and
right down to the jeweler's mark on the back.
So far nobody knows how it got into Eldest's room.
That is the only mystery left. Most of the people
I queried have gotten back to me.