The Delightful Diamond is a jewel that appears in "The Delightful Diamond Mystery".
The Delightful Diamond is small and blue and very shiny. It has two octagon sides and many pentagon faces.
Aside from being pretty, the diamond isn't shown to have any notable traits. It does have the distinction of being Queen Delightful's favorite diamond as the thought of losing it made her cry.
In the show
In the episode "The Delightful Diamond Mystery", the diamond goes missing while the Magic Mirror is asleep. Starchbottom finds out about this in the morning and tells Queen Delightful. She then summon the 7D to help her search. They are clueless about how to catch the criminal until the Mirror tells that crooks tend to return to the scene of the crime. Doc then builds a trap to catch the thief. After a crazy chase sequence, the 7D find the diamond under Starchbottom's bed. But just as Starchbottom's goose seems to be cooked, the trap goes off with Sir Yipsalot inside. Doc is puzzled by this and the Queen wants him released. But just as Doc opens trap after saying "I'll fetch him out", Sir Yipsalot quickly snatches the diamond and runs out of the room. The 7D chase him back to his stash which has many round objects, including the diamond. It's revealed Sir Yipsalot took it because Queen Delightful was teaching him to fetch round things. He gets (slightly) punished for his behavior and the diamond is returned.
The Delightful Diamond briefly reappears in "Mirror, Mirror" in its rightful place.