The Apollo astronauts brought back a magnetized moon rock, one of the oldest and most beautiful rocks with bright green and milky white crystals. Scientists believe the rock proves that 4.2 billion years ago, the moon had a dynamic liquid core that produced a magnetic field about one-fiftieth as strong as Earth’s that lasted for millions of years. It fits the theory of the moon forming after a Mars-sized body hit Earth, sending a lot of Earth’s crust into space, amassing to form the moon.