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Opal still contains 6 to 10 percent water, a remnant of that ancient sea. Opal is one of the few gemstones that are sedimentary in origin. The other kinds of opal include Fire Opal, which is transparent to translucent, a blue opal known as Peruvian Opal, and Girasol opal, which comes from Mexico mainly, and exhibits a bluish glow or sheen that follows the light source around which is not a play of color. Therefore, only approximately 0.25% has any real value at all. The remaining 5% has some color, but about 95% of it is of mediocre grade. About 95% of all opal mined from the opal fields are common or potch, basically one colored. When common opal is found in association with precious opal, it is known as potch. Common opal does not exhibit a play of color.

Common opal is dull and valueless and occurs in abundance throughout the world. Precious opals exhibit the characteristic called “play-of-color,” and are comparatively rare. There are two main varieties of opal - precious and common. In other words, the bright rainbow look of opal, as they appear to the when seen, will move and change dramatically depending on the angle at which the stone is viewed. Its most unique character is that it displays all the colors of the spectrum resulting from the interference and diffraction of light passing through it. Unlike other gems, opal comes in many varieties – white, blue, red, black, and even colorless. nH2O, mixing silicon dioxide with water.To define opal in layman terms, it is a solid gem mineraloid (has mineral characteristics, but is not one) that is solidified from the chemical silicon dioxide, which come from cavities in decaying vegetation, wood, and bones. All black opal comes exclusively from Australia. More than 90% of the world’s quality gem opals come from Southern Australia, although it can be found in other parts of the world such as Ethiopia, Brazil, Mexico, Czechoslovakia and Nevada. The minerals bubbled up from beneath the surface of the earth and slowly over centuries lined the walls of cavities in the bedrock. Opal tends to be found near the earth’s surface in areas where ancient geothermal hot springs once flowed. Most opal is more than 65 million to 145 million years old and is found in the Cretaceous layer of rock which was during the period when dinosaurs roamed the earth. The chemical composition of opal is SiO2H2O, silicon dioxide combined with water (an opal stone may contain up to 30% water.) Opal’s Moh’s hardness rating is measured at 6.0 to 6.5 on the scale similar in hardness to quartz. Opal differs as it is formed from amorphous lumps of silica rather than that from naturally faceted crystals. Opal is related to the more common crystalline cousins such as quartz and agate. Opal is a non-crystalline form of mineral silica.