The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon. We find ultralow friction and wear for ultrananocrystalline diamond surfaces even in dry environments, and observe negligible rehybridization except for a modest, submonolayer amount under the most severe conditions (high load, low humidity). This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond.