So basically I had this encounter with a pretty girl at a store. Basically I Found a leather wrist strap and wanted to know the price, and decided to use it as humor material.

Me: Hey is this for a girl or boy?
Her: It's for a Boy
Me: Are you sure, your not lying me are you? lol
Her: Nope.
Me: I mean it seems like some kinda ruff girl would wear this
Her: Haha
Me *Puts on Bracelet* Now I look ruff with it
*She's still laughing*

I say, every piece of jewelry I wear has a story, like what's your first impression of my ring (it's a Fluer di Liss) she said it's Nice. I said thank you, it's French, it's from their military.

I said, "Ok, now I'm gonna see what kinda piece of your jewlelry I like.. *looks* I like your necklace what's the story on that. Her: It's just a gift for my birthday. Me: *Jokingly* Oh, what a boring story.

Eventually I think I got dull or bored her.

So basically, I sorta joked too much, and I think she sorta loss interest in me. She seemed really tired I caught her just before closing. So what did I do wrong here or SHOULD have done to get the girl?

Personally I don't pickup staff because they are kind of 'trapped' so it's hard to tell how much they are interested how much they just have to be polite because it's their job.

Maybe you could have tried to number close her or asked what she was doing after work/at the weekend? Not sure where else you could have gone with that to be honest.

Your conversation with her had a strong aura of "agenda". She's not stupid, she felt you wanted something out of her.
If she's 'sure' is like saying 'hey, I got no follow-up to my previous line, so I'll just put this shit in'.
She laughed because you were more of a circus-clown than a funny, interesting guy that approached her.
And here you were trying to force a reaction out of her. You're not guiding her into conversation, you're telling her "TALK TO ME. COMMENT ON MY JEWELRY, PLEASE".
After trying to make her invest in asking you something, you're now trying to make her invest in freely talking about something personal about herself. And when she replies nicely, you try punishing her for it by making a rather rude joke.
All of this could have been avoided if you'd approached her from a different angle. You were trying to go under the radar, but it wasn't discreet at all. I'd suggest you try doing direct game to avoid triggering the bullshit-radar for her and to avoid engaging in most of the PU bullshit this community teaches.