Maria get their dark appearance from the basalt rocks that make up their surface. The highlands of the Moon in contrast to the maria are much lighter in appearance, they are made of lighter rocks known as anorthorsites. All of the rocks found on the Moon are igneous rock which means they were formed from molten lava. However the fact that the maria are made of basalt which is formed when molten rock cools rapidly and the highlands of anorthosites which forms when molten lava cools slowly means that two areas of the moon formed under very different conditions. The different conditions required to make these two kinds of rocks suggests that these areas on the moon formed at different times.
Nasa image of a sample of rocks returned from the Moon by the Apollo 11 mission. Many of the early probes and Apollo manned missions helped us to learn about the surface of the Moon and the materials it was made up of. The Apollo 11 mission was the first mission to return samples of Moon rock to the Earth for detailed examination.