A diamond crown is an item in Evony TKR (The King's Return).
Diamond 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 Diamond Crown fragments for gold.
The description of Diamond Crown in Evony TKR says:
Crowns worn by the nobles. The important exhibits in Noble Crown Hall.
Diamond 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.