Perfectly spherical sand to clay size particles composed of magnetite occur in a wide variety of environments. Within the greater New York City area such spherules are found in smoke-stack filtrate, urban-air filtrate, ditritus collected from highway surfaces, lake sediments, marsh sediments, and in beach sands. The abundance of spherules in such environments diminishes away from industrial centers. The abundance of magnetite spherules in core samples of lake and marsh sediment decreases sharply near the palynologically determined stratigraphic boundary separating sediments deposited before the area was settled and industrialized from sediments deposited afterward. Their abundance in samples of deep ocean-floor sediment also drops off sharply within the upper 10 cm of core. The spherules become rare to absent below this depth. We conclude that the great majority of magnetic spherules presently accumulating in sediments are of industrial origin, while we recognize the existence of spherules of probable extraterrestrial origin in ancient sediments. The ease of identification and recovery of magnetite spherules from sediments facilitates their use as indicators of probable post-industrial age.