Our Moon consists of rock, which was formed newly during its creation through a collision between the Earth and another planet, around 4.5 billion years ago. Lunar maria were created through meteoroid impacts, which can be seen as dark spots if observed from Earth.
A small amount of these rocks are on Earth. One part stems from a variety of moon missions (Apollo and Luna), the other from meteorites that struck the Earth as fragments. If you add up the registered mass of these rocks, you’ll end up with 738 kg (1,627 pounds). Which is actually »just a grain of rice« in comparison to the 81,000,000,000,000 kg that our Moon weighs.
Moon rock is ancient. It dates back to between 3.2 and 4.6 billion years ago and is considerably older than the material that can be found on the Earth’s surface. Hence, it is of great use for scientists, because it reaches back to the earliest period of the solar system and can give some indication of its development.
Should you ever have the rare opportunity of being offered a piece of real moon rock for sale, you have to dig deep into your pocket. The prices for 1 g could be in the region of up to 1 million dollars. For interested parties, it might be worthwhile creating a »carpool« to the Moon. Incidentally, one should not confuse moon rock with moonstone, a mineral, which received its name from its white color. This would be a bit more affordable …