From the Wikipedia entry on Libyan desert glass:
The origin of the glass is a controversial issue for the scientific community, with many evolving theories. Meteoritic origins for the glass were long suspected, and recent research linked the glass to impact features, such as zircon-breakdown, vaporized quartz and meteoritic metals, and to an impact crater. Some geologists associate the glass not with impact melt ejecta, but with radiative melting from meteoric large aerial bursts. If that were the case, the glass would be analogous to trinitite, which is created from sand exposed to the thermal radiation of a nuclear explosion. The Libyan desert glass has been dated as having formed about 26 million years ago. It was knapped and used to make tools during the Pleistocene Era.These two specimens were sold by Crystal Circle, via Bijoux et Mineraux.