Per request, here is a video of the light spectrum that is created by hanging cut crystals in a window in sunlight. This is the image you get if you are looking into the crystal from the rainbow it projects. The original post is here. These are fun decorations, and each crystal can create dozens of rainbows if the lighting is right.
This photo is a better example of the rainbows that a crystal makes. You can see all the way from a deep red to violet, and all the steps in between.