A work of art is often a synopsis of its time. Versailles tells of 17th century French rationalism in its orderly facades and the geometry of its gardens. Michelangelo's sculpture reveals in its robust anatomy the renaissance of man's faith in himself. Yet few objects compact so much of a world into a microcosm as the Romanesque cross recently acquired by Manhattan's Metropolitan Museum of Art.
The Met first heard of the cross eight years ago; it had been stashed away in a Swiss bank vault by an Austrian collector. It was carved from seven pieces of walrus tusk, a distinctly North...