Opal – Birthstone For October – Where Do They Come From? One of the opal’s most enchanting features is its captivating play-of-color, an optical phenomenon that emerges when light refracts through the silica spheres embedded within the stone. This play-of-color creates vibrant flashes of rainbow hues, capturing the imagination and evoking interpretations of divine signs across various cultures. The size of these silica spheres, which can range from 5 to 300 nanometers, directly influences the spectrum of colors seen, thereby enhancing the opal’s mystical reputation. In contrast, the milky or waxy appearance of common opal lacks this dynamic interplay of hues, resulting from larger, more irregular silica particles. Read more.