While the colorful glory of the opal is undeniably striking. Opals range in color from white to black, with glints of yellow, orange, green, red and blue. They derive their name from the Greek word Opallos, which means “to see a change (of color).” Opals were formed by non-crystalline silica gel that seeped into crevices within sedimentary strata. Over time the gel hardened to form opals. They’re essentially composed of particles closely packed together in delicate arrangements. When these particles are packed together in a regular pattern, they create a three-dimensional array of spaces – which create the opal’s unique radiance.
For more info: http://www.gia.edu/opal