Precious Opal the birthstone for October
You may know this opal by other names. Such as white opal, solid opal, solid white opal or light opal In its higher grades it is often called clear or crystal. This variety is usually formed in a silica called Potch. Potch is defined as "Common non precious opal without diffracted colors!" Potch is normally the host rock that the opal was cut from and a small amount remains on the surface or inside the stone after cutting.
Opal was formed millions of years ago, when silica and water, mixed together, flowed into cracks and spaces in the ground, then gradually hardened, solidified and became opal. Most of Australia’s opal fields were formed about 100 million years ago. Rain would dissolve some of the silica present in the earth’s surface and the silica-laden water collecting on the groun then filled up cracks, joints and cavities. When the water evaporated, the remaining silica formed a gel made up of minute silica spheres. The gel occurred as thin veins, sheets or nodular masses which today is found as opal seams or as ‘nobbies’ (small nodules, sometimes containing color). After gradually solidifying over a hundred million years of weathering it became hydrous silica; basically silica which has retained a small water content, which to us is better known as opal. Sometimes, the silica mixture replaced organic material, such as wood fragments, shells or bone, so that opalised fossils are not uncommon.
The color of opal does not come from any inclusion in the stone, but is caused by the diffraction of light. Where the minute spheres of silica are of a uniform size and arrangement, the light reflecting from them is split into its spectral colors, and the stone appears to contain all the colors of the rainbow. Where the spheres are larger and less uniform, the range of color is less or nonexistent.
The chemical formula for opal is SiO2.nH2O and its hardness is between 5½ and 6½ on the Moh's hardness scale. Opal naturally has a water content, which varies quite considerably, but is usually between three and ten percent.
Opal is found Mainly in New South Wales, South Australia and Queensland. All of theses areas are mined by either individuals or small groups of miners. Without doubt, Queensland has the brightest and widest variety of Opal in the world, some with breathtaking electric colors. Lightning Ridge in N.S.W has the only true Black Opal mostly found in nobbys (small nuts). Seam opal is found in areas surrounding Grawin and Glengarry. White Cliffs in N.S.W produces the prettiest light crystal opal. Coober Pedy in South Australia is the largest light opal producing field in Australia.