ROME (AP) — One was a delicate, tasseled pendant, featuring a flawless 10-carat diamond surrounded by a ring of rubies and diamonds. The other was simpler but no less gorgeous: two pear-shaped 30.2-carat diamond earrings surrounded by smaller, irregular-cut diamonds suspended like starbursts.

The Al Thani Collection on Friday revealed details of the precious jewels stolen Wednesday from a Venetian palace exhibit in a brazen theft that sorely embarrassed Italian security officials. A manhunt has been launched for the thieves, who calmly pocketed the gems and mixed in with the crowd to make thei...