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  Encyclopedia of Keywords > Nature > Natural Resources > Minerals > Opal   Michael Charnine

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  1. Opal is a hardened gel of silica and water.
  2. The opal is the official gemstone of South Australia.
  3. Opal is a unique gemstone with a beautiful play-of-color display.
  4. Opal is a true precious stone which occurs in many varied forms.
  5. OPAL is a state-of-the-art 20 megawatt pool reactor which uses low enriched uranium fuel and is cooled by water.

Black Opal

  1. The largest black opal in the Smithsonian Museum, possibly worth in excess of $1 million, comes from the Royal Peacock Opal Mine in the Virgin Valley.
  2. So, inclusion of the term black opal was considered to be an imperative.
  3. If the tone of the opal appears darker than N4, then the opal may be classified a black opal (Fig.


  1. Opal ranges from colorless through white, milky blue, gray, red, yellow, green, brown and black.
  2. The veins of opal displaying the play of color are often quite thin, and this has given rise to unusual methods of preparing the stone as a gem.
  3. Close up of opal colour pattern.
  4. Some opal has a coloured body tone, such as yellow, orange, red or brown.
  5. Precious opal is composed of a network of submicroscopic spheres of aqueous silica, separated by voids.

Precious Opal

  1. Precious opal is called dark or light, according to its body tone (the background to its play of spectral colour).
  2. The greater part of the world's supply of precious opal comes from the Coober Pedy and Andamooka fields in South Australia.
  3. A light yellow altered wood occurs in Virgin Valley, Nevada, together with precious opal log replacements.


  1. When such opal is translucent and attractively coloured, it is sometimes cut in cabochons or even faceted.
  2. The rough opal, although it may look as if it will cut well to produce valuable stones, may have faults within it.
  3. Broken fragment might be confused with quartz, but opal's lesser hardness is a good guide.
  4. More from Rock & Mineral Guide Abbreviations information about opal - The source for acronyms and abbreviations.


  1. Contrarily, opalescence is correctly applied to the milky, turbid appearance of common or potch opal.
  2. The other two stop codons were named 'ochre" and "opal" in order to keep the "color names" theme.
  3. Many specimens have a high water content, and as a result, have a greater tendency to desiccate and crack than most precious opal.
  4. Opal is amorphous silica with a water content varying from one to twenty percent, depending on the porosity and degree of hydration.
  5. The Virgin Valley Opal Fields Of Humboldt county in northern Nevada produce a wide variety black, crystal, white, and fire opal.

Common Opal

  1. Potch is a miners' term for a grey common opal found on the Australian opal fields.
  2. Common opal has no play of colour, although it can be attractively coloured yellow, brown, green, blue or pink by other materials.
  3. Common opal is usually milky white, grey or brown but can be coloured yellow, pink, blue or green by traces of other material.


  1. The body tone of an opal is different to the play-of-colour displayed by precious opal.
  2. Opal can be opaque, translucent or transparent, with or without play of colour.
  3. A full range of colours swirls and flashes in the depths of a light opal.
  4. Body tone refers to the relative darkness or lightness of the opal, while ignoring its play-of-colou r.
  5. These must be disclosed as synthetic opal composites (Fig.

Lightning Ridge

  1. Lightning Ridge is the home of Black Opal, the most precious variety of all opals.
  2. Lightning Ridge and Coober Pedy are the places where light opal is mined in Australia.
  3. In 1908 opal mining begins at the Grawin-Sheepyard Field in the Lightning Ridge area, increasing the importance of the opal fields in the district.

Boulder Opal

  1. Australia is the 'real' home of Opal today and the most important supplier of Black Opals, Crystal Opals and Boulder Opal.
  2. Some boulder opal possesses this body tone, so it is very correctly termed black boulder opal.
  3. Essentially there are three types or forms of natural opal, which are termed simply opal, boulder opal and matrix opal (See Figs.

Fire Opal

  1. Common, water, jelly, and fire opal are found mostly in Mexico and Mesoamerica.
  2. Fire opal is a bright red transparent or translucent opal that may or may not show a play of color.
  3. Hyalite is a transparent, colourless opal that looks like glass Fire opal, found in Mexico, has a vivid orange body colour and often displays play of colour.


  1. Fig. 14. A mosaic dark opal composite.
  2. The key to classification as crystal opal is really the transparency of the opal (See Fig.
  3. Mosaic and Chip Opals - are a composition of small flat or irregularly shaped pieces of natural opal cemented as a mosaic tile on a dark base material (Fig.


  1. An opal doublet is a thin layer of colorful material, backed by a black mineral, such as ironstone, basalt or obsidian.
  2. Although the potential value of the opal can be estimated while the material is unprocessed, its actual value cannot be established at this stage.
  3. Doublet Opals - are a composition of two pieces where a slice of natural opal is cemented to a base material (Fig.


  1. In practical densitometers the bulky integrating sphere was replaced with a piece of diffusing opal glass.
  2. Opal is not a kind of quartz, but like quartz, it is made of silica, which is silicon dioxide, which can also be a glass.


  1. The shape, size, and alignment of these spheres affect the color of the opal.
  2. The colours seen in precious opal are dependent upon the size of its spheres.
  3. Australia produces 95 per cent of the world-s supply of precious opal.


  1. On Mohs’ scale of hardness, opal is 5.5-6.5.
  2. When buying an opal, make sure the brightness is 3 or more on the scale.


  1. A high percentage of the opal found there occurs in thin layers.
  2. It occurs on seams in clay-iron stone boulder opal.


  1. Opal can also form in cavities in volcanic rocks.
  2. The sludge escapes through the mesh in the sides, leaving only pieces of rocks, and hopefully opal.
  3. Opal is formed at low temperatures from silica-bearing waters and can occur in fissures and cavities of any rock type.
  4. Different types of opal have been given descriptive names, many of them coined on the opal fields.


  1. Opal displays a burst of striking colors known as play of color.
  2. These "polymer opal films" belong to a class of materials known as photonic crystals.
  3. Precious opal shows a variable interplay of internal colors and does have an internal structure.
  4. The background colour may be white for white opal or light blue for light crystal opal is translucent and shows colours sharp and visible below the surface.
  5. Opal also may be treated to change its natural appearance or durability.


  1. An example of Hungarian Opal seen on this image, found in the 17th century in Hungary, measuring 130mm x 70 mm x 70 mm and weighing 594 gramms.
  2. This type of opal is called common opal.
  3. Hungarian opal is a milky (white) type of opal, far inferior in quality compared to Australian opals in fire and brilliance.
  4. The term -solid- has been removed from opal terminology, for the simple reason that all types of opal are essentially solid from a scientific point of view.


  1. Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix.
  2. This opal is commonly known as matrix opal.


  1. A nice crystal opal, transparent weighing 1.10 cts.
  2. In August 2006, as part of further commissioning, the first uranium was loaded into the new OPAL reactor.
  3. The OPAL reactor is already earning dividends for Australian science by attracting bright young scientists from around the world.


  1. Dislocations occur in the regularity of the opal structure, causing colour boundaries.
  2. Properties of opal Opal's physical properties vary with differences in its structure and water content.


  1. The N9 category is referred to as white opal (See figure 1).
  2. After examining current industry standards, the N4 category was decided to be the cut-off point for black opal.


  1. Precious opal usually contains from six to ten percent water.
  2. From this vague tale, opal developed a reputation for bringing bad luck and the sales of opals dropped 50 percent.


  1. These provide the gemstone industry as a whole with a logical and unbiased way of grading and evaluating opal.
  2. The Opal Association strongly endorses the use of these definitions with the aim of standardising nomenclature across the opal industry worldwide.


  1. Common opal is truly amorphous, but precious opal does have a structural element.
  2. That is, opal does not exist naturally either as a liquid or a gas.


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  Originally created: August 11, 2007.
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