The chemical formula of Opal is indicated by SiO2 – nH2O or Hydrated Silicon Dioxide. Opal is actually a mineraloid and it is usually found exhibiting nice and interesting microscope images when evaluated with the aid of petrographic polarizing light microscope used in the field of optical mineralogy. Opal is most commonly used for gemstone purposes and it is also used as ornamental stone. Opal has been a popular gem for many centuries. Opal is known to have a very interesting structure as studied in optical mineralogy. Opal is classified as mineraloid because its structure is not truly crystalline. The chemistry of Opal has varying amounts of water. Despite the fact that Opal has no crystal structure or regular arrangement of atoms, it still does possess a structure. The name Opal is said to have been derived from the Sanskrit name for precious stone, upala. It has been mined for centuries, at least since Roman times when they extracted the Opal from areas now within Czech Republic. However, not all Opal are considered precious.

 

            Opal is most commonly found white, colorless, pale yellow, pale red, gray or black when impurities are common as found when specimen is closely evaluated under polarized microscope used in the field of optical mineralogy. It has been understood in the field of optical mineralogy that diffraction can actually cause flashes of any color of the rainbow or play of light. Opal is most commonly found showing a vitreous luster when specimen is viewed in reflected light of polarized microscopes for mineralogists. Most specimens of Opal are found transparent to translucent in appearance. Crystal system does not apply because Opal is amorphous.

             The crystal habit of Opal as described in the field of optical mineralogy usually include massive, cavity-fillings such as in fractures and geodes, nodular, reniform or as replacement of other minerals and wood. Opal has absent cleavage even when specimen is closely evaluated using the Mohs scale method. Opal is also most commonly found showing conchoidal fracture when viewed under polarizing microscopes for mineralogists. The hardness measure of Opal when specimen is evaluated using the Mohs scale method is usually ranging from 5.5 to 6. Opal is also most commonly found leaving a white streak when specimen sample is rubbed on a white porcelain streak plate. The specific gravity measure of Opal is approximately 2 to 2.5 grams per cubic centimeters, which is considered light. Most Opal specimens fluoresce white or pale green color in ultravilote light. Some other specimens phosphoresce. All specimens, however, can be very sensitive to impacts and low temperatures. The best field indicators of Opal includes color play and opalescence, lack of cleavage or crystal faces, low density, fracture filling tendency and fluorescence. Opal is usually found associated with cherts, a form of microcrystalline quartz. It is also associated wit volcanic rocks and many others. Most precious Opal comes from Australia as well as Mexico and the Western United States. Opal notably occurs at several localities including Australia, Czech Republic, England and in many Western USA localities.