BOSTON.- As the saying goes, diamonds are a girls best friendat least in modern timesbut as the exhibition Jewels, Gem, and Treasures: Ancient to Modern illustrates,ornaments made of ivory, shell, and rock crystal were prized in antiquity, while jewelry made of diamonds, emeralds, sapphires, rubies, and pearls became fashionable in later years. On view July 19, 2011, through November 1, 2012, this exhibition at the Museum of Fine Arts, Boston (MFA), highlights some 75 objects representing the rich variety of jewels, gems, and treasures that have been valued over the course of four millennia. Drawn from the MFAs collection and select loans, these range from a 24th-century BC Nubian conch shell amulet, to Mary Todd Lincolns 19th-century diamond