What are Saturn's rings made of?

Ice and rock, in countless separate pieces. Some are grains of dust, some are the size of a house, and every one of them is travelling on its own orbit around the planet.

From far away they look like solid bands, and they are nothing of the kind: they are more separate things than anyone could ever count, moving together in silence. The rings stretch enormously wide, yet in places they are only about as thick as a house.