Hello there, folks.
In this solution we are attempting to get you the valuable(and the Achievement for grabbing it) that can be found in the Royal Stones Historic Land Contract. I will include a text description as well as a a timestamped video below that includes all 4 Ancient Britain Valuables. These Valuables can only be found in Career Mode, not in the Free Mow section available from the Main Menu.
When you load up the contract, you'll see the LARGE gathering of stones slightly in front of you to the right. Walk forward and you should see a dirt path show up to the right of these stones. Start following it and look in the field on your right. There is a smaller circle of stones over here. Run to them, and on the rock closest to your spawn point in this circle there is a Valuable.
The Royal Stones location begins at 1:49