The name opal derives from the Greek Opallos, meaning "to see a change (of color)." Opals range in color from milky white to black with flashes of yellow, orange, green, red, and blue. An opal's beauty is the product of contrast between its color play and its background. Opal is a formation of non-crystalline silica gel that seeped into crevices in the sedimentary strata. Through time and nature's heating and molding processes, the gel hardened into the form of opals. The opal is composed of particles closely packed in spherical arrangements. When packed together in a regular pattern, a three-dimensional array of spaces are created that give opal its radiance.