Opal is made of the same ingredients as quartz, except it contains a little water and has not been compressed into crystals. As a result, it is softer than quartz and has to be treated a little more carefully to avoid damage. The shifting colors seen in opal, called "fire", are the result of microscopic spherical structures within the stone that reflect different wavelengths of light depending on their spacing, creating the colorful shimmering effect. There is no other gemstone that looks remotely like it. It comes in both black and white varieties, with black being the most rare. Australia is the principal source of opal today. Like other non-transparent stones, it is usually cut into a smooth, rounded, polished dome called a cabochon.
Opal ranges from transparent to opaque. Its value is based on the "play of color" with red being preferred as the dominant color. Cabochon cut is the preferred shape to enhance the play of color. A black background with color play is the more valuable black opal. Boulder opal is a stone that has veins of opal running through the host rock (iron matrix).
Some legends say that opal is good for the eyes, both improving vision and warding off eye troubles. It also has a reputation for sharpening the mind and the emotions. Opal is a symbol of fidelity and is said to bring trouble to someone who is unfaithful.
Opal is October's birthstone.
||Iridescent white (and sometimes black)
||Semi-translucent silicon dioxide with patches of many colors showing through. Mohs' hardness rating of 6.