The firm responsible for the divine artistry of this circa 1920 one-of-a-kind platinum, emerald, and diamond brooch, which sold at Sotheby’s April 17 Magnificent Jewels auction for $2,909,000, remains a mystery.But its winning characteristics are obvious: For starters, the brooch features “top-quality stones,” says Catharine Becket, vice president and senior specialist of Sotheby’s jewelry department. In the center is a 22.48 ct. emerald framed by five marquise-shape diamonds. “Your eyes are drawn to the exquisite emerald right away,” says Becket. “There is no clarity enhancement, which is incredibly rare for a stone of this size, and unique depth of color.”The symmetry and geometric nature of the suspending marquise, pear, and baguette diamonds perfectly complement the ­emerald, suggesting that it emerged from the atelier of a fine maker. As for its lack of signage, in