Courtney Robertson Returns Engagement Ring -- Who Said Diamonds Are Forever?

Courtney Robertson is about $80k lighter today -- because "The Bachelor" winner finally returned her diamond engagement ring to its rightful owner ... and photogs caught every glorious moment of it.

Robertson -- who called it quits with "Bachelor" Ben Flajnik last week -- hit up a Neil Lane jewelry store in L.A. Sunday to say sayonara to the 3-carat rock.

We're told the ring ended up back at Neil Lane since "Bachelors" and "Bachelorettes" don't foot the bill for their own marital swag -- i.e. when the relationship goes away, so does the bling.

But Court's not shedding too many tears over the loss (of the ring or Ben) -- considering she was recently seen sucking face with "Bachelorette" cast-off Arie Luyendyk Jr.

Ah, love.