The authors of the paper in Science say it was meteors.

At each site the scientists looked at, the diamond layer in the rocks correlates to the date of the hypothesized impact. Within the layer, the scientists report finding a multitude of diamond particles, all encased within carbon spheres. “We’ve yet to find a single diamond above it,” Dr. West said. “We’ve yet to find a single diamond below it.”

Image Scientists found microscopic diamonds in the black layer of rock at Murray Springs in Arizona. Credit... University of Oregon

Perhaps more telling, the scientists reported last month at a meeting of the American Geophysical Union in San Francisco, the carbon atoms inside some of the diamonds are lined up in a hexagonal crystal pattern instead of the usual cubic structure. The hexagonal diamonds, formed by extraordinary heat and pressure, have been found only at impact craters and within meteorites and cannot be formed in forest fires or volcanic eruptions, Dr. West said.

Last year the scientists presented other evidence of an impact, including elevated levels of the element iridium.

At least some skeptics are not convinced. “The whole thing still does not make sense, and there are lots of contradictions,” said Christian Koeberl, a professor of geological sciences at the University of Vienna in Austria.

His chief reservation is that there is no crater. “A body of this size does not just blow up without a trace in the atmosphere,” Dr. Koeberl said. “Physics won’t have it.”

Proponents have suggested that the meteor hit an ice sheet a couple of miles thick or that there was a series of smaller objects that exploded in the air. But Dr. Koeberl said something hitting an ice sheet would still generate a hole in the ground underneath, and he questioned whether smaller impacts or air explosions would produce the shock waves needed to make diamonds.