When did Earth's crust form?

It's easy to take the ground beneath our feet for granted, but the story of our crust's origins is one of the most hotly debated topics in Earth history. Today, we'll learn how elements inside tiny crystals help solve this mystery. On the way, we'll meet two researchers who helped keep gold and scientists out of German hands in World War II.

Extra credit: Look at table salt under a microscope, or find different colors of quartz in your local mineral collections.