“This is an early model from the 20th century made in platinum—as opposed to silver and gold from the 19th century. It is lighter and thinner and more delicate.”    Chaumet’s enamel and diamond tiara, which sold for $676,104 at Christie’s Geneva on Nov. 10, combines two great headwear styles: the kokoshnik, the traditional Russian headdress renowned for its distinctive shape and intricate floral designs; and the classic diamonds popularized by British royalty and seen on red carpets. “The rarity of such an important jewel, its good condition, and its provenance appealed to a lot of potential buyers and created strong competition in the auction room,” says Jean-Marc Lunel, senior international jewelry specialist for Christie’s. “The translucent blue enamel is both rare and in good condition, contributing to the selling price that was $150,000 over estimate.