In certain gemstones, notably opals, brilliant areas of color can be seen within the stone; these areas change in hue and size as the stone is moved. This phenomenon, known as play of color, differs from fire and is caused by interference and reflection of the light by tiny irregularities and cracks inside the stone. Opals also exhibit milky or smoky reflections from within the gem. Gems that are fibrous in structure show irregular interior reflections similar to those seen on watered or moiré silk. This optical property, which is called chatoyancy, is exhibited by several gems, notably the tigereye and cat’s-eye.