SOME BASICS TO HOW OPAL IS FORMED!!!
A silicon dioxide and water combination is used to create opal.
Silica from sandstone is picked up by water as it travels through the earth, carrying it into holes and fissures left by natural faults or decaying fossils.
A coating of silica is left behind as the water evaporates.
After many, many years of this cycle's repetition, opal is finally produced.
Smaller spheres tend to defract the purple, blue, and green colors and are more frequently forming. This explains why these hues are less valuable than those of yellow, pink, red, and orange. Larger spheres are more uncommonly formed and typically exhibit the colors yellow, orange, pink, and red. This explains why these are the harder to find hues in Australian Opal.
THE SCIENTIFIC EXPLAINATION!!!
When conditions are right, silica spheres from the earth's silica-rich fluids occasionally form and settle under the force of gravity to form layers of silica spheres. At a depth of forty meters, the solution is thought to deposit at a rate of about one centimeter in five million years. When the technique enables the spheres to grow to a uniform size, beautiful opal starts to form. The size of the spheres in precious opal spans from roughly 150 to 400 nanometers, which results in a play of color via diffraction in the 400 to 700 nanometer visible light range.
For opal deposition to occur, each local opal field or occurrence needed to have had voids of some kind. Opal appears to only fill gaps in volcanic rocks and their surrounding surroundings, whereas weathering has left a variety of voids in sedimentary rocks. In addition to the pre-existing fractures, open centers of ironstone nodules, and horizontal seams, leaching of carbonate from boulders, nodules, and various fossils also creates a variety of molds suitable for the deposition of secondary minerals such as opal.
The majority of the opal deposit is not priceless. The mineralogists refer to it as common opal since it lacks a color play, but the miners refer to it as "potch." Opaline silica not only fills the larger voids mentioned, but it may also fill the pore space in sediments of silt and sand size, binding the grains together and forming distinctive deposits, such as matrix, opalized sandstone, or "concrete," and is a more conglomeratic unit close to the foundation of early Cretaceous sediments.
Numerous variables affect the numerous variances in opal varieties. A rising or, more significantly, falling water table that concentrates any silica in solution is caused by the climate's alternating wet and dry seasons. The silica itself is produced by extensive weathering of Cretaceous clay deposits, which also produces white kaolin, which is frequently found in association with the Australian opal fields. In order to create the distinct environment necessary for the development of its own variety of opal, special circumstances must also exist to stop a declining water table.