If it is GIA certified, it may have a number etched into the side of the diamond, which would tell you who purchased it and where it came from. 1/2 carats of good cut, color, and clarity will bring upwards of 350 to 600 dollars. I just purchased one from Diamonds.com for my wife, .65 carat for 730 dollars.
Interesting story, wife and I were in the bar, crazy busy, my wife tells me she lost one of her diamonds. Our friend starts looking, bumping into people, she proceeds to reach down in the carpet and pick up the diamond. This was only like a .4 carat diamond. I quess her beer goggles work that night.
Balzac, from your posts I am sure you will do the right thing. Good Luck