A royal treasure so highly valued dat you'll so totally get da electric chair if you're caught while attempting to steal it.
If you wanna avoid da "crown joules" while touring da Tower of London, you should TAKE only a GOOD LOOK, not try to TAKE any of the exhibits themselves.
The absolute very best of something. Typically, an adjective that refers to something that is the best it can possibly be based on qualities of being exceptional or nothing but the best.
My professor is the crown jewel of the English department; she's the very best and the most knowledgeable.
The Vespa Granturismo Vintage is the crown jewel of scooters.
Jordans are the crown jewel of the sneaker world.