jewel (n.) Look up jewel at
late 13c., "article of value used for adornment," from Anglo-French juel, Old French jouel "ornament; present; gem, jewel" (12c.), which is perhaps [Watkins] from Medieval Latin jocale, from Latin jocus "pastime, sport," in Vulgar Latin "that which causes joy" (see joke (n.)). Another theory traces it to Latin gaudium, also with a notion of "rejoice" (see joy).

Restricted sense of "precious stone, gem" developed in English from early 14c. Figurative meaning "beloved person, admired woman" is late 14c. Colloquial family jewels "testicles" is from 1920s, but jewel as "testicle" dates to late 15c. Jewel-case is from 1753.