Opals

When water permeates the earth it becomes rich in dissolved silicates. Eventually silicates will enter a cavity and form microscopic spheres, creating opals. If the spheres are of equal composition they will diffract light producing the iridescence effect of precious opals. In contrast, if the silicate deposits form in random arrangements the result is the common opal. Common opals may have a waxy or opaque luster and are often fluorescent, whereas precious or noble opals have a distinct fire color.