Opal

The opal is a commonly occurring mineral from the mineral class of " oxides and hydroxides ". As an amorphous solid opal has ( similar to glass ), no crystal structure and usually occurs as massive vein filling or bulbous formed.

Opals are processed exclusively for gemstones.

• 5.1 Education 5.1.1 Sedimentary Opals formed
• 5.1.2 Volcanic opals formed

• 5.2.1 Australia and Oceania
• 5.2.2 Africa
• 5.2.3 America 5.2.3.1 North America
• 5.2.3.2 South America

• 6.1 Opal assessment 6.1.1 Composite Opals, Synthetic Opals, Other

Special Features

As one of the few amorphous minerals consisting of hydrated silica opal with the general chemical composition SiO2 • nH2O. The water content is usually between four and nine percent but may reach a maximum of about 20%.

The outstanding feature that makes the opal so desirable as a precious stone, is the buntfleckige iridescent play of colors, the so-called opalescence of Edelopals. It is caused by reflection and interference of light beams between the beads of silica gel, which are between 150 and 400 nanometers in size. In this precious opal Kieselgelkugeln are approximately equal and are regularly arranged and densely packed before.

The similar-sounding and often misleading term used opalescence on the other hand refers to a milky - bluish, the pearlescent similar optical effect of the common opals.

Opals can be dissolved by hydrofluoric acid and potassium hydroxide. Opal melts when heated over an open flame is not, but is dull and crackles.

Etymology and history

Opal The term is taken from the Latin or the Greek opalus ὀπάλλιος opallios for, precious stone ' and probably originates from the Sanskrit upala. However, this is doubted by Allan W. Eckert in his work The World of Opals, because this word applies to all rocks, while the early Magyars / Hungarians designated opal mines as Opalbanya.

Opals were already in antiquity as particularly valuable gems that have been evaluated in some cases even higher than that of diamond. Pliny the Elder wrote: " [ ... ] it is proper to a fire, a fine in the Carbunculus, he has the spark of purple amethyst and the sea green of the emerald and an ever incredible mixture of light. "

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the opal belonged to the mineral class of " oxides and hydroxides " and then to the Department of " oxides with the molar ratio of metal: oxygen = 1: 2", where he, together with coesite, cristobalite, melanophlogite, Moganite, quartz, stishovite, tridymite, the " quartz group " was formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the opal also in the class of " oxides and hydroxides " and there in the department of " Metal: Oxygen = 1: 2 and comparable " one. This division, however, is further divided according to the size of the involved cation and the family relationship of the minerals and the crystal structure, so that the mineral "With small cations: Silica family," according to its composition in the subdivision is to be found where there are only still together with tridymite forms the unnamed group 4.DA.10.

In contrast to the Strunz'schen classifications predominantly common in English-speaking classification of minerals according to Dana Opal belongs to the class of " silicates and Germanates " and there in the department of " framework silicate minerals " one. Here he is the only member of the unnamed group 75.02.01 within the subdivision " framework silicates: tetrahedral Si lattice, with H2O and organic molecules SiO2 " to find.

Varieties

The silica gel itself is colorless or at most slightly. Colorless or water-clear varieties are accordingly called glass or milk opal. Through different colored impurities of Opal can but all sorts of darker colors, also called body color, accept. Gray, amber, red, brown and black varieties rarely come before. In the jewelry industry, a distinction between precious opal and Commons opals.

Angel Skin Opal, however, is a misleading name for an Opal like mineral called palygorskite from white to pinkish color.

A spectacular but extremely rare variety, the so-called " Pineapple Opale " (also Opal Pineapple ), hedgehog or pineapple -shaped pseudomorphs of Opal by Ikaite that were previously found only in opal fields near White Cliffs and Andamooka in Australia.

Opal

General opals are designated with vivid, opalescent play of colors in gemstone quality than precious opals. In contrast, Common Opal, called potch in Australia, no play of colors. A special form the fire opals from Mexico, which show a uniform transparent color without play of color and are among the precious opal.

Trade names

The main occurrence of the rare black opal at Lightning Ridge is located (New South Wales, Australia ), especially in the opal fields Coocoran, Gravin and Glengarry. Other Locations in Australia are the Mintabie in South Australia, further Mexico, Nevada, in the United States and Indonesia. A distinction is beside the Black Opal Black Opal crystal, which is transparent to a certain degree. In addition, there is the Dark Opal, which remains dark even with transmitted light, and the light gray opal. The deposits are located mainly in Mintabie and are found in places where black opals occur.

Light Opals were formerly called White Opals. The deposits are located in South Australia at Coober Pedy, Mintabie, White Cliffs and also in Brazil. The bright Opals are divided into Bright Crystal Opal and Jelly Opal.

A special feature are the opals in Queensland, Australia, which are summarized below Boulder Opal. There Boulder Matrix Opal, which is connected to the limonitic rock and Yowah Nut Opals, which are enclosed by an iron-containing shell. Bright, Dark and Black Boulder have the appropriate color play in the rock. There is also the Boulder split, a delaminated in stock Opal direction with the same color pattern.

The fire opals from Mexico ( Jalisco in Magdalena ) and Brazil (Rio Grande do Sul in Campos Burgos and Piaui in Pedro II ) is available with or without play of color; with color play are usually faceted to trade.

The Dendritenopale, also called Moosopale are characterized by a wide range of colors. You are in Lower Austria ( Dobersberg ), Czech Republic ( Cesky Krumlov ), Zambia ( Lake Kariba ), Peru and found in Western Australia at Norseman. You rank with existing colors to match the precious opal.

Very popular Harlequin Opal, with its strong, segmental play of colors in transparent to translucent ground substance.

After localities opals are also often called, such as the Lightning Ridge black opal, Mexican black opal, Andamooka - crystal opal or Andenopal.

Light Opal

Matrix Opal

Fire Opal (Mexico)

Common Opal

The Commons opals counts of Hyalite (from Greek hyalos = glass) or Glasopal, with the grapey - nieriger to crust- shaped design shows a simple, water- clear opal no play of colors.

The also the Commons opals attributed Hydrophan ( from the Greek hydor (water) and phanos ( seem ) ) or milk opal is caused by age-related loss of water from opal and is milky white with just a matte luster and faint opalescence. It is due to water absorption for a short time again transparent and receives his full colors. The stone was called in the old mineralogists also world eye ( oculus mundi ). For use as a jewelry he has no meaning, but you can use it in a liquid as a decoration.

Wood opal finally came through silicification of wood and is yellowish to brownish in color.

Other belonging to the Commons opals varieties are the banded Achatopal, the reddish to brown by iron compounds Jaspopal, the yellowish Honigopal, the porcelain-like, pearlescent Kascholong ( Cacholong, Porzellanopal ), the Moosopal with its dendritic structures, the green and often as an imitation of the chrysoprase Prasopal used ( Chrysopal ) and the yellowish-brown, resinous luster Wachsopal.

As siliceous sinter or geyserite loose, fine-grained opal crusts are called, which settle through the activity of hot springs and geysers.

Diatomaceous earth is the only variety used technically. As a loose aggregate with fine pores it is very absorbent and heat insulating and is therefore used in construction.

Education and Locations

Education

Opals are formed by siliceous fluid accumulation in different rocks. They occur either in Sedimentiten or hydrothermally in volcanic rocks such as tuff in, but also by sedimentation in organic material, thereby producing among other wood opal. Accessory mineral is of Chalcedon.

Sedimentary opals formed

In sediments and sedimentary rocks formed by slow water loss a silica gel that turns into a solid consistency. During the evaporation of the water involved therein, there remains a residual component. These processes determine the way the sedimentary Opalbildung. Alternating cycles of wet and dry climatic periods as well as the weathering products of existing clay minerals are further condition Opalbildung. The great Australian Opalvorkommen that originated in a sedimentary basin, the fifth of Australia covered, are now seen as a result of weathering processes in which silicic acid solutions in clays, sands, gravels Cretaceous rivers and sandstones as well as to fine-grained, clay-rich marl layers and coarse-grained conglomerate bands invaded. Opal could fill pore spaces in granular sedimentary structures, which may also be revealed as cementation. In this way, just as crowded existing lineaments, voids in fault zones and other planar discontinuities in rocks. As these spaces were traversed by groundwater and climatic periods increased and sank the ground water level, the therein silica gels could settle and consolidate.

The colors of the opal (black, white, gray, blue, green and orange ) - regardless of the color play, also depend on the chemical composition of the surrounding rocks and the trace elements contained therein such as iron, cobalt, copper, nickel, silver, etc. from.

The colors of the Edelopals is determined by layers of myriads of tiny pebbles spheres and their diameters in the nanometer range (nm), which form in the development processes and reflect the light.

Volcanic opals formed

Opals are also found in volcanic rocks, for example, the fire opal from Mexico in a Rhyolithvorkommen. The occurring in volcanic rocks opals were created by hydrothermal processes in which heat and pressure play a significant role. In studies of Russian scientists, it was found that the formation of hydrothermal opals in two ways different to the sedimentary: The silica spheres at the nanoscale are not parallel, as in the formation of sedimentary opals, but embedded chaotic. In contrast to the three-dimensional balls of the sedimented opals two-dimensional incomplete photonic bands arise in the chaotic opal matrix. Block -like structures and thin films are responsible for the spectral color and iridescence volcanically formed opals.

A special feature is the Eibenstockopal at Eibenstock in Saxony, which was wire- shaped, embedded in an igneous rock, granite layers. This also applies to the Forcherit in Austria at Ingering in Styria, which occurs in clefts of gneiss, a metamorphic rock.

Locations

Total so far (as of 2013) are over 3000 Locations for Opal to be known. With about 95 % of world trade precious opals, however, Australia is the largest exporter. Mexico holds about 4 % of world trade and the balance of about 1% comes from other parts of the world.

Australasia

• Australia in the states of New South Wales (Lightning Ridge, White Cliffs ), South Australia ( Andamooka, Coober Pedy, Mintabie, Lambina ) and Queensland ( Yowah, Koroit, Toompine, Quilpie, Kyabra - Eromanga, Bulgroo, Yaraka, Jundah, Opalton - Mayneside and Kynuna ), and also in Springsure ( opals volcanic origin ) and Dendritenopale at Norseman in Western Australia
• Indonesia in Java and Sumatra
• New Zealand in Coromandel Valley ( geyserite )

Africa

• Ethiopia ( Opal in Rhyolite )
• Mali at Nioro du Sahel, Djudigui ( opals in volcanics )
• Zambia on Lake Kariba ( Dendritenopal )

America

North America

• Mexico: Jalisco at Magdalen, Querétaro and Hidalgo

In addition to Mexico, there are in North America or in the United States, Canada and Honduras sites of Opal.

• Honduras at Intibukat, Erandique, Tambla and Guayoca
• USA: Arizona at Tucson, California in the Mohabe Desert, Idaho in Yellow Stone National Park ( Occurrence: Geyirit and volcanics ), Louisiana at Lafayette, Nevada in Virgin Valley, Oregon in the Blue Mountains and Utah at Milford

South America

The only Edelopalvorkommen South America are in Brazil, in Peru, the blue and pink so-called " Andenopal " is terminated.

The largest Brazilian Fund area is the area around the place Pedro II in the state of Piauí. The milli - to centimeter-thick Opalgänge fill cracks between sedimentary rocks from the Devonian and Jurassic dolerite. Due to the low water content of the opals from Pedro II are among the toughest and most stable in the world. They are commercially mined since the early 1970s, the most important mine was and since the mid- 2000s again the Boi Morto - Mine. This was built and operated by Australians, for mining explosives and heavy machinery were used. From the late 1970s, the flow rates of up to 50,000 kg decreased annually, In 1985, the last mine in Pedro II Local Garimpeiros investigated further by hand at the surface for opals and always found smaller reserves funded with amounts from a few kilograms. The mid-2000s opened an Australian Opalhändler the Boi Morto - mine again and has since been pursuing a mechanical degradation by. Brazilian fire opal was found in Castelo do Piauí and the state of Rio Grande do Sul, there in the communities Espumoso and Campos Borges.

Peruvian opals are collectively referred to as " Andenopale ", but the two variants differ significantly from each other. The blue opals (also blue - green, blue - gray and turquoise ) are conveyed in the Acarí copper mine near the town of Nazca from a five -centimeter-thick layer. This is largely interspersed with brown and black dendrites, only individual opals are of clear blue color. The color is caused by microscopic chrysocolla inclusions and the Opal thus contains up to one percent copper. Pink Opal comes from the Monte Rosa mine near Ica and is not a pure opal, but a mixture of opal, chalcedony and palygorskite. The color is caused by small amounts of quinones, organic compounds contained inter alia in vegetable dyes.

Asia

• Kazakhstan at Wosnjesjenskoje and Aleksejewskoje
• Turkey, Western Anatolia in Karamandja at Simav and in western Turkey Dendritenopale

Europe

Dubník in Slovakia was probably best known in Roman times as a mining area for opals. Secured However, the Opalabbau is from the 15th to the 19th century. 1920 this reduction was discontinued altogether. There also one of the largest precious opals in Europe was found in 1670. The 594 -gram Opal is since about 1672 in the Imperial Treasury in Vienna and is considered the most precious gemstone collection in Vienna ( Naturhistorisches Museum in Vienna).

• France (named Quincyt, according to the site Quincy ) between Tours and Bourges and in the Plateau Central and in the Paris Basin ( in the latter, the dark brown Leberopal )
• Germany in Saxony at Gröppendorf and yew floor ( historical archaeological site )
• Iceland ( geyserite )
• Austria at Ingering in Styria ( Forcherit, a rare opal which is found in the clefts of gneisses )
• Slovakia ( as Dubník to 1920 in Hungary was a historical site where the so-called Hungarian opals ) at Dubník
• Czech Republic ( since Červenka was a historical site where the Hungarian opals to the year 1920 in Hungary) at Červenka and Český Krumlov ( Dendritenopale ).

Outside the earth

NASA was 2008 that there are large areas on Mars that contain opal. From the presence of opal is concluded that there once reigned conditions that could have allowed an evolution of life forms.

Use as a gemstone

Opal gem quality are extremely rare, so a industrial mining takes place only in a few places in the world. Opal is indeed found on all continents, but there are about 95 percent of all opals from Australia. There, the majority of the worldwide promotion deliver the deposits to the town of Coober Pedy.

In order to bring its bright color play to unfold, opals are smoothed to Cabochonen. An exception is the fire opal, where the red glowing luster is reinforced with a bevelled edge.

Opal - assessment

According to this classification have Natural Opals (English: Natural Opal) neither saw marks still polishes. There are three types: Type 1 is homogeneous in chemical composition and type 2 is the typical boulder opal from Queensland, which is still attached to the rock in which it arose and a different chemical composition. Its outer sides can be opalescent. Type 3 was in rock bands or in cavities or he has flitterartige storage in rock and Matrixopal is called.

Other criteria are body color and transparency.

The body color varies from relative obscurity to brightness. An assessment ignored the play of colors and is based on the AGIA Body Tone Chart. Black Opals can be clear transparent or opaque. According to the scale opals are further sorted into Dark and Light Opals Opals. Opals with a unique yellow, orange, red or brown color are classified as either Black, Dark or Light Opal and according to the AGIA Body Tone Chart.

Opals are different transparent to opaque. This is detected in transmitted light. Transparent to slightly transparent opals are called Kristallopale. Crystal means here only a name, because opals are amorphous.

Composite opals, synthetic opals, Other

Kompositopale consist of natural opal laminates, manually cemented that are on other materials. There are three types, doublets, triplets and marquetry.

Since the Black opals are rare and expensive, doublets and triplets are produced. Opaldubletten consist of calibrated precious opal in the millimeter range on a dark background, mostly on dark chalcedony or Potch. Opaltripletten build up of three layers, consisting of a layer Commons Opal as a base, about a millimeter thinner opal and it earlier to protect a transparent rock crystal, glass or lead glass hard today. The Opaldubletten and triplets imitate Black Opals. Inlays are composed of small precious opal, which are cemented onto a base to symbols, patterns and motifs.

Natural opals can be changed by color, heat, coloring, background, glue, varnish, waxes, oils or by the use of chemicals in their appearance. There are also synthetic opals, which are produced artificially, but have an identical composition as the opals, and also made ​​of plastic, resin and glass.

Esoteric

Already in ancient Rome was the opal as the stone of love and hope. He is generally regarded as a talisman of thieves and spies. The mystics of the opal is used as a healing stone for increasing vitality and optimism. In addition, it is designed to relieve or generally act against inflammation physical ailments such as strep throat. Even with heart and kidney problems Opal is said to have healing effects. The increase of vitality is, however, also can be reversed, as the Opal allegedly intensified every feeling. Instead vitality therefore can also be a possible fatigue life be intensified. Scientific evidence for the alleged physical and mental effects do not exist.