Moon Rock is a unique type of rock, found only on Equestria's moon.
Moon Rock was first brought to Equestria's surface by the first pony space rocket that went to the moon. Astronauts collected three kilograms of moon rock, before returning to Equestria's surface. The material was heavily studied by the O.I.A. and other scientific organisations, which discovered its unique properties and the risks it posed.
Moon Rock is harmless by itself, the material appearing to be unremarkable, white rock. When Moon Rock comes into contact with star metal, specifically the kind found in Hoofington, it emits massive bursts of harmful radiation and explodes with incredible force.