1 Get the red gemstone
Go to the west wing of the second floor of the mansion. Enter the big room in the corridor that has the staircase that leads to the safe room. Solve the eagle puzzle to get the Red gemstone and also the yellow one.
2 Go to the mansion's main hall
Go to the main hall of the mansion and use the helmet key to open the door on the north east of the hall. Note that you won't have the helmet key before you return once again to the mansion.
3 Get the jewelry box
Once inside that room, follow the corridor until you reach the other room. Grab the jewelry box after killing the zombie in the room.
4 Combine the items
Combine both the Jewelry box with the red gemstone in order for the box to be opened.
5 Solve the box puzzle
Once you combine the elements, you will be presented with a puzzle where you will have to arrange broken pieces in order to open the box.
6 The key is inside the box
Once you solve that puzzle, you will find an emblem which will turn into a key upon clicking on it.