If you thought the styles from the 2000s were dunzo, you were wrong -- 'cause Kristin Cavallari is bringing 'em back with a splash ... alongside her "Laguna Beach" crew.

The businesswoman is launching a special jewelry line through her Uncommon James brand -- and while it's new -- it's also kinda old in a sense ... or rather, nostalgic. She and her team are calling it the Back to Laguna Collection, and it's chock full of pieces from the aughts.

Here's the best part ... Kristin actually got together with former 'LB' co-stars to shoot a promo for this retro campaign, which features all your fave personalities from the show. Yes, that includes  ... Talan Torriero, Jason Wahler, Alex Murrel, Jessica Smith and Alex Hooser.

Not just that -- but KC's ex-boyfriend, Stephen Colletti, is also in this thing. How about that?!

Indeed ... it's a welcome stroll down memory lane -- and you can see it on the 'LB' stars' faces here. FWIW, we're told they had a helluva time filming this as well ... they reunited in December at Pacific Palisades High School, where the good times (and cameras) were rolling.

Sources tell us that in between takes and during breaks, Kristin and co. were yakking it up like the old days -- as many of them hadn't seen each other in years. Pretty darn neat.

Now, as for what's included in this new Back to Laguna collection ... well, it's good stuff. A lot of these jewelry items are things you might've seen -- or even worn -- back in 2004 ... like the iconic black cord choker, gold rings (for all your fingers), earrings, bracelets and necklaces that reflect the aesthetic from back in those days.

In other words ... all the accessories you used to love as a teen and/or early twentysomething are back and for sale -- all courtesy of Kristin and Uncommon James -- plus a little help from her old friends.

It doesn't get more Orange County than this, folks.