It's Patek's first and only Grandmaster Chime in steel. More importantly, it's now the world's most expensive watch. Recently sold for over $31 million at the Only Watch charity auction, Patek Philippe ref. 6300A-010 is a one-of-a-kind masterpiece. Its 48mm reversible case is made from stainless steel with a guilloched hobnail pattern and holds two dials. One, rendered in rose gold, tells the time, date, and moon phase, while the opposite ebony dial displays calendar information. In all, it features 20 complications, including five acoustic functions and two world premieres: an alarm that strikes the preprogrammed time and a repeater that sounds the date on demand. It's finished with a hand-stitched shiny black alligator leather strap and is unlikely to hit the market again anytime soon.