On a recent Saturday afternoon, visitors wandered through Sotheby’s, on the Upper East Side, to inspect articles featured in this season’s auctions. On the ground floor, they could inquire about a bottle of Château Mouton Rothschild 1945 from the wine cellar of the businessman William Koch. In the exhibition halls upstairs, a Mayan classical-period stucco head was on display, as were a collection of Signacs and two diminutive Francis Bacon self-portraits. And, on the sixth floor, in a small room outside the jewelry section, surrounded by images of the cosmos and encased in glass, a truly unusual object was rotating atop a velvet-covered pedestal: the Lesedi La Rona, the biggest diamond in the world. At eleven hundred and nine carats, the stone was about the size of a fist. Its skin was opaque in places, but certain angles offered brief, clear glimpses into the depths of the crystal: a tightly woven, perfectly symmetrical carbon lattice.