Distinguishable due to its voluminous rounded case, the Ballon Bleu is so-called thanks to the blue sapphire crown that accents the bezel. First released in 2007, it's now considered to be one of Cartier's most sought-after styles. This pre-owned Ballon Bleu 42mm combines the French maison's contemporary vision with the traditional Roman numeral dial that identifies its timepieces.