The most revealing thing we discovered when we surveyed the reigning engagement ring styles of the past 150 years? How many would be coveted—and considered marketable—today.   Victorian The discovery of diamonds in South Africa in 1867 floods the market with rose-, old mine–, and old European–cut stones. Snake motifs—popularized by Queen Victoria herself—are de rigueur, as are colored gemstones such as turquoise, opal, sapphire, ruby, garnet, and amethyst. In 1886, Tiffany & Co. introduces its famous Tiffany setting, which features the diamond perched high above the finger with six unobtrusive V-shape prongs securing it in place. The setting effectively makes the gem, not the metalwork, the star of the show. Victorian rose-cut diamond cluster ring in 14k gold; $1,835; foxandbo