A gem crown is an item in Evony TKR (The King's Return).
Gem Crown is one of the collection items in the exhibition for the Art Hall. It is one of four parts in the Noble Crown Hall. After the set is completed at three stars, you can trade additional Gem Crown fragments for gold.
The description of Gem Crown in Evony TKR says:
Crowns worn by the nobles. The important exhibits in Noble Crown Hall.
Gem Crown is known to potentially be obtainable from opening Artwork Fragment Chest I, Artwork Fragment Chest II and Artwork Fragment Chest IV. The links for each item are below:
Note: This list may be incomplete.