Purists might be tempted to put quotation marks around the word pearl when referring to the rare conch pearl, owing to the gem’s lack of a nacreous coating, but there’s no doubt that in a value hierarchy, the pearlescent product of the queen conch (pronounced konk) would top the pyramid. Found primarily in the waters surrounding the Bahamas and southern Florida, the mollusk produces what’s known as a calcareous concretion. Don’t, however, let the science obscure the beauty of these refined gems, on full display in this exceptional double strand of multicolor pearls, which sold for nearly two times its high estimate at Sotheby’s Magnificent Jewels sale in New York City in December. The strand, say experts, defines the very limits of rarity. “These pearls aren’t available,” says Gina Latendresse, owner of American Pearl Co. in Nashville, Tenn. “We could count the number o