The opal is the birthstone of October, and is the most precious gem variety of silica known to man. Its distinctive colors are caused by diffraction of light as it passes through tiny spheres in the opal's structure. The play of color is the extraordinary phenomenon seen in precious opals, where the surface of the stone scintillates and flashes with all the brilliance of a diamond. Some opals exhibit this phenomenon to an exceptional degree. Even when viewed by transmitted light, many precious opals have a metallic or oily appearance caused by their high content of water which acts as a brilliant diffusing agent for white light. The play of color is the extraordinary phenomenon seen in precious opals, where the surface of the stone scintillates and flashes with all the brilliance of a diamond.
