Kate Middleton’s wedding tiara was impressive, but it was nothing compared with this mid–Victorian headpiece. The former ­property of Hannah Primrose, Countess of Rosebery, it fetched $1.9 million at Christie’s ­London Important Jewels sale in June. “That the tiara survived in the original form was remarkable,” says Keith Penton, F.G.A., D.G.A., head of jewelry at Christie’s London. One possible reason: convertibility. “It could be dismantled to form a series of cluster brooches with hanging pearl drops, or a less elaborate tiara of circular clusters, leaving the drops to be attached to other jewels.” Also, he adds, “the Rosebery family was extremely wealthy and didn’t need to refashion their jewels. They could simply acquire more!” The price tag was not, however, about provenance, but rather, the pearls. “In the last five to 10 years, natural pearls have again