crown jewel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A precious stone that is part of a sovereign's regalia.
• noun The jewels, such as those in a crown or scepter, used ceremonially by a sovereign.
• noun The most prized asset or possession in a group.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the most desirable assets of a corporation
• noun a precious stone that is a valuable part of a sovereign's regalia