Tektites are naturally occurring objects which have never satisfactorily explained by scientists worldwide.
Currently the most popular theories speculate that Tektites have an extraterrestrial origin. That's not to say they were brought here by little green men, but who knows? It's generally believed they come either directly from the Moon, or from the effects of large meteorites hitting the Earth.
Each Tektite is different, but they're usually 2-3cm long and come with an information card.