Opals on Blue Placemats byceciliamok

The uniquely spectacular opal is the national gemstone of Australia. The internal structure of precious opal makes it diffract light. Depending on the conditions in which it formed, it can take on many colors. Precious opal ranges from clear through white, gray, red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black. Opals vary in optical density from opaque to semitransparent and show opalescence, a form of iridescence. Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids , caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time, and eventually opal is formed. Opals are made up from tiny uniformed spheres of transparent hard silica. It is the order of the spheres that separates the precious opal from the common opal. These tiny spheres also determine colours and patterns of opals. The size of the spheres has a bearing on the colour produced. Smaller spheres bring out the blues, larger spheres produce the reds. And more and more uniform the spheres are placed, the more intense, brilliant and defined the colour will be.