Properties of Opal
- Chemical Composition - Hydrated silica (SiO2 Ā· nH2O), a mineraloid.
- Hardness - 5.5 to 6.5 on the Mohs scale.
- Density - 1.98 to 2.25 g/cmĀ³.
- Color - Exhibits a play of color, ranging from reds and oranges to blues and violets.
- Transparency - Ranges from opaque to semi-transparent.
- Internal Structure - Composed of silica spheres in a hexagonal or cubic lattice.
- Fracture and Cleavage - Conchoidal fracture with no distinct cleavage.
- Specific Gravity - Approximately 1.9 to 2.3.
- Lustre - Vitreous to resinous.
- Optical Properties - Diffracts light, resulting in the play of color.
- Stability - Sensitive to temperature changes, may crack with extreme heat. Avoid prolonged sunlight exposure to prevent dehydration.
Found in various types of rocks, including limonite, sandstone, rhyolite, marl, and basalt, opal is a versatile gemstone. However, it is most famously associated with Australia, which accounts for an impressive 97% of the world's opal supply. The national gemstone of Australia, opal, takes center stage in the jewelry and gemstone market.
Opal's color palette is as diverse as its geological origins. Ranging from clear and white to red, orange, yellow, green, blue, magenta, rose, pink, slate, olive, brown, and black, opal displays a remarkable array of hues. The rarest combination is red against a black background, while white and green opals are more common. The gemstone's optical density varies from opaque to semi-transparent.
The internal structure of precious opal, despite being a mineraloid, is composed of silica spheres in a hexagonal or cubic close-packed lattice. These spheres, with diameters of 150 to 300 nm, create an ordered microstructure that causes the diffraction of light, resulting in the gem's stunning play of color. The regularity of the sphere sizes and their packing defines the quality of precious opal.
Bragg's Law of diffraction explains how the spacing between the silica spheres' planes determines the colors observed in opal. This phenomenon creates an optical band gap, akin to a photonic crystal. However, visible light of diffracted wavelengths cannot penetrate large thicknesses of opal, contributing to its unique optical properties.
Opal's play of color is often seen in thin veins, leading to unconventional gemstone preparations. Opal doublets, for instance, consist of a thin opal layer backed by a darker mineral such as ironstone or basalt, enhancing the play of color. In contrast, opal triplets have a domed cap of clear quartz or plastic on top, producing a more artificial appearance.
Apart from precious opal, there are common opal varieties like milk opal, wood opal, menilite, hyalite, geyserite, and diatomite. Each variety possesses distinct characteristics, adding to the overall allure of opals.
Varieties of Opal
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| Opals Of The World, Photo: Opal Auctions |
Opals, renowned for their mesmerizing play of colors, come in various types, each with unique characteristics. Let's delve into the diverse world of opals.
Black Opal
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| Black Opal Photo : Flickr/James St. JohnĀ |
Originating primarily from Lightning Ridge, New South Wales, black opals boast unmatched brightness of color due to a dark body tone. Contrary to common misconception, "black" doesn't mean entirely black; it refers to a dark body tone. Ranging from dark grey to jet black, Australian black opals are highly valued for their vibrant spectral colors.
White Opal
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| White Opal Photo : Flickr/James St. John |
Also known as "milky opal," this variant features light white body tones and is mined in South Australia. While more common than black opal, its body tone may not showcase colors as vividly. Nevertheless, high-quality white opals can exhibit stunning hues, offering a captivating alternative to their darker counterparts.
Boulder Opal
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| A boulder opal |
Forming on ironstone boulders in Queensland, boulder opal often retains the ironstone on its back. This characteristic contributes to its dark and beautiful coloration. Found within cavities of boulders, these opals vary widely in size and shape, with a tendency to cleave naturally, revealing two polished faces.
Crystal Opal
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| 7.75 ct. Lightning Ridge Crystal Opal Photo : Mardon Jewelers |
Any opal with a transparent or semi-transparent body tone falls under the category of crystal opal. This type can exhibit both dark and light body tones, leading to distinctions such as "black crystal opal" and "white crystal opal."
Fire Opal
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| Fire Opal |
Transparent to translucent, fire opals showcase warm body colors ranging from yellow to orange to red. While not displaying the typical play of color, some may exhibit bright green flashes. Originating notably from the state of QuerƩtaro in Mexico, these opals are occasionally referred to as Mexican fire opals or jelly opals.
Girasol Opal
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| Girasol Opal |
Often confused with fire opals, true girasol opal is a type of hyalite opal. It displays a bluish glow or sheen that follows the light source, resulting from microscopic inclusions. Found in Oregon and Mexico, it is also known as water opal in Mexican varieties.
Peruvian Opal (Blue Opal)
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| Peruvian Opal |
Semiopaque to opaque, Peruvian opal is a captivating blue-green stone found in Peru. It is sometimes cut to include the matrix in the more opaque stones. This type does not exhibit pleochroism. Blue opals are also found in Oregon's Owhyee region and Nevada around Virgin Valley.
Mexican Water Opal
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| Mexican Water Opal |
A colorless opal from Mexico, exhibiting either a bluish or golden internal sheen. It is sometimes referred to as Peruvian Opal when from Peru.
Australia's dominance in opal production is unparalleled, with 97% of the global supply originating from the continent. Coober Pedy, located in South Australia, stands out as a major opal source, producing various types, including the world's largest gem opal, the "Olympic Australis."
The Mintabie Opal Field and Andamooka in South Australia, along with Lightning Ridge in New South Wales, contribute significantly to the opal industry. These regions yield crystal opal, black opal, matrix opal, and boulder opal, showcasing the diverse opal landscape found across Australia.
In addition to its gemstone wealth, Australia is home to opalized fossil remains, including dinosaur bones in New South Wales and marine creatures in South Australia. The rarest Australian opal, known as "pipe" opal, is closely related to boulder opal and forms in sandstone with iron-ore content, often as fossilized tree roots.
Australia produces around 97% of the world's opal, and 90% of it is called 'light opal' or white and crystal opal. White makes up 60% of the opal productions but cannot be found in all of the opal fields. Crystal opal or pure hydrated silica makes up 30% of the opal produced, 8% is black, and only 2% is boulder opal.
The town of Coober Pedy in South Australia is a major source of opal. The world's largest and most valuable gem opal "Olympic Australis" was found in August 1956 at the "Eight Mile" opal field in Coober Pedy. It weighs 17,000 carats (3450 grams) and is 280 mm long, with a height of 120 mm and a width of 110 mm. It is valued at AUD$2,500,000.
The Mintabie Opal Field located approximately 250 km northwest of Coober Pedy has also produced large quantities of Crystal opal and also rarer black opal. Over the years it has been sold overseas incorrectly as Coober Pedy Opal. The black opal is said to be some of the best examples found in Australia.
Andamooka in South Australia is also a major producer of matrix opal, crystal opal, and black opal. Another Australian town, Lightning Ridge in New South Wales, is the main source of black opal, opal containing a predominantly dark background. Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. It is found sporadically in western Queensland, from Kynuna in the north to Yowah and Koroit in the south. Its largest quantities are found around Jundah and Quilpie (known as the "home of the Boulder Opal") in South West Queensland. Australia also has opalized fossil remains, including dinosaur bones in New South Wales, and marine creatures in South Australia. The rarest type of Australian opal is "pipe" opal, closely related to boulder opal, which forms in sandstone with some iron-ore content, usually as fossilized tree roots.