This slice is of a Moon rock that landed on Earth after ejection from the lunar surface following an asteroid impact. There are only 275 kilograms (600 pounds) of lunar meteorites known to exist—with a significant fraction held in institutional colections—and they would all fit in two large suitcases. Lunar meteorites are identified by specific mineralogical, chemical, textural and radiation signatures. The Moon is among the rarest substances on Earth. Many common minerals found on Earth’s surface are rare on the Moon and some lunar minerals are unknown on Earth. In addition, Moon rocks contain gases captured from the solar wind with isotope ratios that are very different from the same gases found on Earth. While Apollo astronauts returned with less than 400 kg of Moon rocks, not one milligram is available to collectors.