Moonstone claddagh rings display a beautiful optical effect known as iridescence. More exactly the effect is adularescence also known as " shiller".This effect causes the lovely blue hue that appears across the moonstone claddagh.
This is a play of rainbow colored light caused by very thin layers and structures beneath the surface of the gemstone. This can be described as being like a thin layer of oil on water which interferes with reflected light rays. This effect is brilliantly displayed in precious opal claddagh rings.
The cause of opal's play of color was unknown until the the 1960s when an electron microscope to explore the structure of the gemstone. It was discovered that opals play of color is caused by millions of sub microscopic spheres of cristobalite. They color reflected white light through light interference and diffraction.