I have a crystal pendant hanging in one window that throws a rainbow out across the room whenever the sunlight strikes it at a certain angle. It is a very pretty effect.
Above is the rainbow it casts on a coffee table.
Above is a Gallery of the changing rainbow colours as sunlight strikes the crystal.
As I have said in a previous post, Scattering Rainbows, you can split white light (sunlight) up into its constituent colours using a glass prism, (or crystal, as here). This was first done by Isaac Newton.