Alloys of terbium are often exhibit magnetostriction, or a change of shape in different magnetic fields. An alloy of terbium, iron, and dysprosium known as Terfenol-D is used in the SoundBug device to turn any solid surface into a speaker. Terbium ions exhibit both strong paramagnetism and bright green fluorescence, and terbium doped phosphors are used in cathode-ray tube televisions to produce green light, along with europium doped phosphors that produce red and blue. Terbium is one of the few rare earths that exhibits multiple air-stable oxidation states.