Gadolinium
Its main uses are as chemical compounds in color televisions and nuclear magnetic resonance radio contrast agents.  In color cathode ray tube televisions (the type first produced, not flat televisions) some of the pixels are dots gadolinium compounds. When the electrons from the cathode ray tube hit the  pixels and transfer energy to the gadolinium compounds they give out light as energy.  Nuclear magnetic resonance (NMR) imaging is used in medicine where it is named MRI (Magnetic resonance imaging).  It is used to take picture of inside the body.  How the pictures look is changed by the gadolinium compounds and how much water is around.  It makes the images clearer because it makes the difference in the concentration of water in different parts of the body look more clear (more contrast) in the picture that is made.