Opal is unlike most gemstones in that its flashing color is not due to the color of the stone itself, or even to the color of its included impurities. Rather, it is due to the way in which tiny opal particles are grouped during its formation. Detailed photographs taken through an electron microscope show clearly how precious opal is deposited as spheres that are so small they are indistinguishable under powerful optical microscopes. These spheres are packed together in very orderly networks, row upon row and layer upon layer, with tiny open spaces, also in rows, between them. Masses of common opal lack this orderly internal arrangement of spheres. White light striking the precious opal is reflected independently by each row of spheres, much like the reflections from a series of slats in a venetian blind. Since these rows of spheres are spaced at distances approximately the same as the wavelength of light, a phenomenon known as _diffraction_ occurs. The separate reflections interfere with each other in an organized manner, cancelling out some of the light wavelengths and reinforcing others, producing color. The brilliant color flashes are of different hues depending on the sizes of the spheres of opal and, therefore, the distances between rows. To provide the best display of this optical effect, opal is almost always cut in cabochon form rather than as faceted stones. [Illustration: Fire opals have rich fire; some have background colors that vary from bright yellow through orange and red; and some are colorless. Stones such as the ones shown here, which weigh 7, 11, and 22 carats, have made Querétaro, Mexico, famous as their source. (Actual size.)] [Illustration: This rare 34-carat opal from Brazil resembles closely the opals found in Australia. (Actual size.)] Common opal, which shows milky opalescence, does not exhibit color flashes, and it is not used as a gemstone. Each of the common varieties—such as hyalite, cacholong, and hydrophane—has its own slightly different set of characteristics, but only precious opal, with its dazzling color display, is important for gem purposes. To take full advantage of the small amounts of gem material available, or to bring out its color better, _precious_ opal is often cut as thin pieces and mounted as doublets on some other backing. Also, the seams in rock sometimes are cut so that the thin layer is exposed on a thicker backing of the adjoining rock. Precious opal, or gem opal, is classified as _white opal_ when the color flashes are in a whitish or light background, _black opal_ when the background material is gray, blue-gray, or black, and _fire opal_ when the background is more translucent and red, reddish orange, or reddish yellow.