|the jewellery, including the regalia, used by a sovereign on a state occasion|
A prized possession or asset, as in The Iliad and Odyssey are the crown jewels of ancient literature, or The software products are the company's crown jewels. This usage transfers the value of royal jewels to some other object. [Late 1800s]
Also, family jewels. The male genitals, especially the testicles. For example, She gave the would-be mugger a hard kick in the family jewels. A slang euphemism, the term dates from the 1970s, and the variant from the early 1900s.