Over the weekend, 21-year-old Robert Driscoll of Trafford, Pennsylvania wanted to propose to his girlfriend, 20-year-old Crystal Yamnitsky.  But he didn't have any money.

So while they were at a party at a friend's house, Robert STOLE that friend's engagement ring.  And then he gave it to Crystal.

She immediately took a photo of it and posted it on Facebook.  She wrote, quote, "Look what Robby gave me.  I love him so much."

But, ya know, when you post pictures of things you stole on Facbeook, other people see them.  One of Crystal's Facebook friends was her cousin . . . and he saw the photo and recognized the ring.

He told the friend who threw the party . . . that friend called the police . . . and they went over to Crystal's house.

When they got there, she told them Robert was out getting the ring resized.  Turns out he'd actually taken it to a pawn shop and sold it for $250.  It was a three-carat diamond ring that was ACTUALLY valued at $12,000.

He was arrested for theft . . . Crystal was arrested for conspiracy and receiving stolen property. 

 Via (NBC 11 - Pittsburgh)