In opalescence, light hits alternating layers of the microstructure of common opal, which creates a shimmering, bluish-whitish color. This color floats below the gemstone’s surface. Most blue opals which come under the category of common opals show opalescence, and some blue opals which are precious opals showcase the play of colors. https://www.sagaciajewelry.com/blogs/news/blue-opal-history-meaning-symbolism-healing-properties-more