Kristin Cavallari Goes Braless in New Jewelry Ad

Kristin Cavallari's haven doesn't have any bras in it ... or, at least her photo shoot for the Haven Collection doesn't -- 'cause her nipples are sticking right through her shirts in the ad.

The reality TV star and internet personality shared a series of photos to her Instagram Tuesday ... wearing a 'fit that left nothing to the imagination -- 'cause it was clear she was feeling a bit chilly without a bra on.

She's smoldering for the camera in these shots ... staring right down the barrel with a ton of confidence -- which ya gotta have a lot of to ditch the support.

The posts are an advertisement for the Haven Collection ... a jewelry collection sold via the brand Uncommon James.

We gotta be honest ... we don't know how many people are tearing their eyes away from the rest of Kristin to focus on the jewelry -- but, at least it's in the pictures!

Cavallari's newest pics come just a couple weeks after she flaunted her bod in a bunch of Stagecoach outfits ... looking like a cowboy everyone would want to ride.