(n) the milky iridescent appearance of a dense clear medium when it is illuminated by polychromatic visible radiation (including sunlight) because of regional fluctuations with its thickness and so in its refractive list
(n) A reflection of a milky or pearly light from the inside of a mineral, such as the moonstone; hawaii or top-notch being opalescent.
opalescence in Community Dictionary
the entire feeling and essence that is generated by Opal, or an opal. Can often be a sense of beauty once the stone is a lovely object. by Mildred HeffronReport definition