Desert Glass
© H. Raab, used via CC license.
Libyan Desert Glass is opaque, greenish glass formed when the desert sands fused in some sort of extremely hot incident. (Alternately, Sandman Volume 2 Number 9 proposes that the glass is the remains of an ancient city.) What, exactly, created the heat that made the glass is a source of scientific debate, but a new paper suggests it might have been the result of a comet impact.

Why a comet and not, say, an asteroid? Scientists studied a stone found in conjunction with the glass and discovered that it contained a mixture of elements that you'd be unlikely to get from an asteroid impact. Instead, the elements suggest an origin outside our solar system's asteroid belt.