Kings and other nobility held fortunes in rare metals and stones (diamonds, rubys etc.). The crown jewels of the monarchy was a symbol of power.
Today every man carries the crown jewels in the crotch of his pants. When did testicles become the crown jewels?
The kid in me can’t help but laugh. The pride and status of the rich & powerful, the crown jewels of the kingdom. Whoever made the term vulgar had a wicked sense of humor. 
Do we know when the vulgar use began and where?
I’ve only heard them called “family jewels” before, and I’d assume that the origin would be pretty obvious.
And that phrase, “family jewels” is attested from 1920. Cite.
I wonder if the Yiddish word schmuck, which means “penis” is somehow related. It’s apparently etymologically unrelated to the German word schmuck, which means “jewelry.”
I’ve only heard “family jewels” too, and “crown jewels” doesn’t refer to the collective wealth or material fortune of a monarchy, but only to the actual jeweled, ceremonial vestments worn by the monarch during formal ceremonies (i.e. the literal crown and scepter, plus whatever rings or necklaces or whatever else they wear when they’re fully decked out for a ceremony).
I’ve heard family jewels too. We always said crown jewels as a kid. Same idea.
But, you’re right, most people say family jewels.
I wonder if some of those who do say “crown jewels” have the image of the orb and sceptre in mind?
Perhaps its rhyming slang.
In the UK they are sometimes called ‘goollies’, not a big stretch to rhyme with ‘jewellry’
Not to be confused with the Groovie Goolies. 
“Jewels” has meant “something highly prized” since the 14th Century.
It seems likely that’s where the metaphor came from.
First you have to provide some evidence that “crown jewels”=testicles in slang. In my experience, it isn’t.
I’m sure you’re just conflating the expression with “family jewels.”
