What Is Opal and What Is It Made Of

処理...

Opal is a non-crystalline form of mineral silica. Opal is related to the more common crystalline cousins such as quartz and agate. Opal differs as it is formed from amorphous lumps of silica rather than that from naturally faceted crystals. 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. 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.

Opal tends to be found near the earth’s surface in areas where ancient geothermal hot springs once flowed. The minerals bubbled up from beneath the surface of the earth and slowly over centuries lined the walls of cavities in the bedrock. 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. All black opal comes exclusively from Australia.

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. In science mode, opal is known as SiO2·nH2O, mixing silicon dioxide with water. Unlike other gems, opal comes in many varieties – white, blue, red, black, and even colorless. 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. 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.

Opal Varieties

There are two main varieties of opal - precious and common. Precious opals exhibit the characteristic called “play-of-color,” and are comparatively rare. Common opal is dull and valueless and occurs in abundance throughout the world. Common opal does not exhibit a play of color. When common opal is found in association with precious opal, it is known as potch. About 95% of all opal mined from the opal fields are common or potch, basically one colored. The remaining 5% has some color, but about 95% of it is of mediocre grade. Therefore, only approximately 0.25% has any real value at all. 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.

Opal Is Sedimentary

Opal is one of the few gemstones that are sedimentary in origin. Opal still contains 6 to 10 percent water, a remnant of that ancient sea. Gold panners in Australia found the first few pieces of precious opal in 1863. A large percentage of the world’s stable commercial opal comes from Australia, and there are three distinctive regions where opal comes from. These are Lightning Ridge (the home of black opal) the South Australian fields Coober Pedy, Mintabie, Lambina and Andamooka (the main sources of light base opal) and the Queensland fields, where boulder opal originates.

Opalescence

The most striking feature of opal is the ability to refract and reflect specific wavelengths of colours. In fact, the term “opalescence” was coined to describe this phenomenon. The size and spacing of the amorphous spheres of silica within the stone refracts specific wavelengths of light; each sphere refracting a single, pure spectral colour much like the droplets of water in a rainbow. The interplay of these pure wavelengths of light gives opal its unique visual appeal and makes it one of the most sought-after gemstones in the world.

History Of Opal

Opals contain the wonders of the skies - sparkling rainbows, fireworks and lightning. Like the sky the opal colours shift and flash with movement, the combination of colours and patterns means every opal is unique. Black opal was first discovered in Australia in the late 1800s. Before its discovery the world was only aware of crystal opal which was typically mined in Eastern Europe. Many royal families throughout history have treasured opal particularly in Europe and Africa. The oldest recorded opal was found by the famous archaeologist Louis Leakey who found six thousand year old opal artifacts in a cave in Kenya! The Roman historian Pliny rightfully described opal as the queen of gems, as opal is a combination of the beauty of all other gems: “There is in them a softer fire than the ruby, there is the brilliant purple of the amethyst, and the sea green of the emerald - all shining together in incredible union. Some by their splendour rival the colours of the painters, others the flame of burning sulphur or of fire quickened by oil.”

Opals have been popular with some of the greatest names in history including Cleopatra and Marc Anthony. One of Marc Anthony’s fellow Roman senators owned an opal stone that reminded Marc of Cleopatra so much that he had to have it. When the Senator refused to sell it to him he sent him into exile.The French Emperor Napoleon was more successful giving Josephine a beautiful opal with brilliant red flashes called “The burning of Troy,” making her his Helen.

Opals were also famous in literature. Shakespeare likening the opals play of colour to play of mind in the most famous use of gemstone symbolism in literature. In Twelfth Night, he writes: “Now the melancholy God protect thee, and the tailor make thy garments of changeable taffeta, for thy mind is opal.” For a few centuries opals seemed to disappear from history. However in modern times Queen Victoria reignited interest in the gemstone. Queen Victoria loved opals and often gave them as wedding presents. She and her daughters created a fashion trend for the wearing of Opal.

Queen Victoria was one of the first to appreciate opals from an exciting new source: Australia. The story of Australian opal begins more than 100 million years ago, when the deserts of central Australia were a great Inland Sea, with silica-laden sediment deposited around its shoreline. After the sea receded and disappeared to become the great Artesian basin, weathering 30 million years ago released a lot of the silica into a solution which filled cracks in the rocks, layers in clay, and even some fossils. Some of this silica became precious opal.

Australian Opal

Opal is predominantly found in Australia. 95% of it, to be exact, is found there, thus making opal the country’s national gemstone. The largest and most valuable opal, the Olympic Australis, was found in 1956 in the town of Coober Pedy, a major source of opal. Other sources of opal included Ethiopia, Nevada, Idaho, and even Mars, where NASA discovered opal deposits in 2008. Ethiopian opal comes from the Wollo and North Shewa Provinces. Wollo opal, also called “Welo” or “Wello” opal, has become the dominant Ethiopian opal in the gem trade. The Smithsonian Institute has he largest unpolished and polish black opals, both from Virgin Valley, Nevada. The unpolished opal is known as the “Roebling Opal,” which was found in 1917 and weighs 2,585 carats. The polish version is called the “Black Peacock” and weights 160 carats.

The major opal producing fields for black opal is Lightning Ridge in New South Wales, Australia, and to a much lesser degree Mintabie in South Australia. The most sought after black opal is red or multicolour on black, which is very difficult to find. Such stones are always cut on the fields and sold immediately. Boulder opal is opal that is formed in a brown ironstone nodule and is distinctive because it typically presents as a thin layer of opal over a dark brown base when cut. The opal can be quite superb. Opal from Lightning Ridge is often considered to be the best and brightest in the world.

Opal is one of the worlds most beautiful and precious gemstones and is one of six types of gemstones used throughout history, along with diamonds, rubies, sapphires, emeralds, and pearls. The word ‘opal’ is adapted from the Roman term opalus, but adapted from the Sanskrit word úpala. It is also the birthstone of October, but, since the 19th century, has been associated with bad luck and death thanks to the novel Anne of Geierstein, which uses opal as a plot device. Regardless, it remains likeable and wanted by gem collectors despite the superstition around it maintain the evil eye, as it is a gorgeous collectors item for enthusiasts of such items.