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Opals are silica-based mineraloids displaying unique optical phenomena. Classification includes natural, composite, and treated forms, further divided by structure and optical properties. Precious opals exhibit vibrant colour shifts, while common opals lack this feature.
Morphological classification separates opals into opal-A, opal-CT, and opal-C. Optical properties such as body tone, brightness, and transparency affect valuation. Regional variations include Australian, Ethiopian, Mexican, Brazilian, Honduran, and United States opals. Black opals, featuring a dark body tone, remain the most valuable variety.
An opal is a unique gemstone defined by its distinctive play-of-colour, a phenomenon where shifting hues of light dance across its surface. This enchanting effect is primarily found in natural opals, particularly in precious opals, which are prized for their vibrant colour patterns.
The internal structure of an opal, consisting of silica spheres in an amorphous form, is integral to its characteristic display. Variations in body tone and base tone contribute to the opal’s allure, with black opals being especially sought after for their dramatic contrast.
The various opal compositions can be classified as natural opals, composite opals, and treated opals, each of which possesses its own distinctive characteristics.
A natural opal is a form of silica (hydrated silica) that has formed naturally over millions of years. Natural opals lack a defined crystal structure, which classifies it as a mineraloid rather than a mineral. The unique play-of-color effect in natural opals is caused by microscopic silica spheres that are arranged in a straight line inside the stone. Precious opal, a type of natural opal, is the most valuable type due to its stunning iridescence, whereas common opals lack play-of-colour.
A composite opal is a manufactured stone combining a thin layer of genuine opals with other materials. Types include composite stones such as opal doublets, opal triplets, and mosaic or chip opals. These enhance appearance and durability while reducing costs. Imitation opals may also replicate this layered construction but do not contain natural opal.
Composite opals often display unique characteristics of body tone, as the backing material can influence the opal’s appearance. In some cases, opal treatments might be applied to further improve colour or stability. Composite opals may contain visible inclusions, unlike solid opals, which are cut from a single piece of natural opal.
A treated opal is an altered gemstone to enhance appearance or stability, often mimicking the black opal range. Treatments like sugar/acid staining or smoke darken the dark stone for a striking play-of-colour. Porous opal forms may undergo stabilisation to resist cracking during contact with water.
The different types of opals are precious opals, common opals, fire opals, hydrophane opals, matrix opals, and synthetic opals.
A precious opal is a gemstone that displays a play-of-colour, a shimmering effect caused by light diffraction within its silica structure. Examples include black opal, white opal, boulder opal, and crystal opal. Precious opals are highly valued for their vibrancy, shifting colors, and rarity.
Common opal, also called “potch,” is a type of opal without the play-of-colour effect of precious opals. Common opals appear in various solid colours, like white, pink, blue, yellow, and green. Valuing its simplicity and charm, it is widely used for affordable jewellery and ornamental designs.
Fire opal is a common opal variety, admired for its fiery hues of yellow, orange, and red. These beautiful stones glow vividly in bright light or direct sunlight and can sometimes resemble jelly opal, known for its translucent, glass-like appearance.
A hydrophane opal is a porous opal that absorbs water, enhancing its transparency, colour, and play-of-colour when wet. Found mainly in Ethiopia, it is unique for its transformative nature but requires careful handling due to its susceptibility to dyes. Hydrophane opals are highly valued for their brilliance.
Matrix opal is a form of opal embedded within a host rock, often ironstone. A key part of Australian opal production, it is similar to boulder opal ironstone. Though it may feature dead spots or chip opals, it remains highly sought after in opal auctions globally.
A synthetic opal is a replica of a natural opal, mimicking its structure and play-of-colour effect. Made from silica or other materials, synthetic opals are affordable, durable, and consistent in appearance, making them ideal for everyday wear.
The types of synthetic opals are lab-created opals and imitation opals, offering a cost-effective alternative to the high black opal prices seen in natural stones.A common type of synthetic opal is lab-created and may exhibit green fluorescence under UV light.
The types of opals by origin include Australian opals, Ethiopian opals, Mexican opals, Brazilian opals, Honduran opals, and United States opals as major globally recognised sources.
Australian opals are gemstones celebrated for their dark body tone, rare patterns like chicken wire and Chinese writing, and varieties including boulder opal and green opals. Mined in areas like Lightning Ridge and White Cliffs, Australia is the largest opal producer, renowned for its world-class opal treasures.
Ethiopian opals are volcanic-formed hydrophane gems that transform from crystal-clear to dark grey when exposed to water. Characterised by deep blues and Harlequin patterns, current Ethiopian opal production rivals Australia’s output. While not common, some specimens display dendritic inclusions, though true dendritic opals and hyalite opal varieties remain rare in these deposits.
Mexican opals are distinguished by their fiery orange-red body colour, forming in rhyolitic rock cavities across Querétaro mines. These gems include transparent fire opal and unique contra luz varieties that display colour play against light through their translucent bodies.
Brazilian opals are crystalline gemstones mined from sandstone deposits in Piauí state’s Pedro II region. These unique specimens form in sedimentary beds, displaying distinct internal structure that differentiates them from other global varieties.
Honduran opals are a distinctive type of opal formed in volcanic basalt, featuring a natural black matrix without artificial treatment. Mined in the country’s western region, these gems include black, crystal, fire, and dendritic varieties, representing the continent’s oldest known opal deposits.
United States opal is found across several western states, with Nevada being the leading producer of this precious stone. While Australia dominates global production, US opals are known for displaying an array of colours, particularly Nevada’s black opals. These gems form when silica-rich water seeps into rock cavities, creating both precious varieties with colour play and common opals without this characteristic.
Opals fall into two main categories. Precious opal displays vibrant play-of-colour effects, while common opal lacks this distinctive feature. They can be categorised as natural solid opals or constructed pieces (doublet opal, triplet opal).
Their appearance ranges from light body tone to black body tone, with black and red being the most valuable colours in both types. Industry experts evaluate these characteristics, along with pattern complexity and clarity, to determine an opal’s value and quality.
The morphological classification system is determined through X-ray diffraction (XRD), which remains the best primary method for delineation and classification of opal types.
The morphological classification system identifies three main categories: opal-A, opal-CT, and opal-C. Within this classification, opal-CT covers a range of structural states, from “simple” opal-CT (including Ethiopian play-of-colour samples) to “complex” opal-CT, which represents the majority of opal-CT samples.
The substrate typology definition is a system that identifies and classifies the base materials involved in opal formation and development. The classification includes natural substrate, artificial substrate, and hydrophane substrate as primary categories. These substrates play essential roles in determining the opal’s physical properties, growth patterns, and overall characteristics.
Natural substrates are the host rocks where opals form, particularly in volcanic rocks that are rhyolitic in composition. These substrates provide available spaces, such as vugs (cavities) in silicic volcanic rock, where opal can form and develop.
In some cases, natural substrates include sandstone seams, where precious opal displays its characteristic play of colours, and rocks where silica-rich groundwater deposits opal in rock pores.
Artificial substrates are laboratory-created foundations used in synthetic opal production through electrophoretic deposition of self-made silica particles. These substrates allow for efficient and cost-effective fabrication of opal structures with controlled uniformity and specific physical properties. The process involves careful arrangement of homogeneous silica spheres in the mesoscale size range.
Hydrophane substrate is a porous base material found in hydrophane opals that demonstrates unique water absorption capabilities. When exposed to water, this substrate becomes translucent or transparent, temporarily altering the opal’s appearance. This characteristic is particularly common in Ethiopian opals.
Optical characterisation subsets in opals involve the analysis of light interaction through various specialised methods. The main subsets include play-of-colour, body tone, transparency, brightness, colour patterns, and spectral range. These measurements help determine the quality and authenticity of opals.
Opal play of colour is a distinctive optical phenomenon that interacts with regularly spaced layers of silica spheres. As an optical characterisation subset, it involves analysing the brilliance and brightness measured by the quality of light returned to the eye from both the surface and within the stone.
The size of the silica spheres affects the colours observed; smaller spheres diffract light towards the blue end of the spectrum while larger spheres lean towards the red end. This phenomenon helps distinguish precious opals from common opals and serves as a key quality indicator in gemological assessment.
Opal body tone is an optical characterisation subset that measures the background colour intensity, ranging from N1 (black) to N9 (white) on the standardised scale. This characteristic helps classify opals into categories like black opal (N1-N4), dark opal (N5-N6), and light opal (N7-N9). As an optical characterisation parameter, body tone influences how other optical phenomena are displayed and perceived.
Opal transparency is a key optical characteristic that ranges from crystal clear to completely opaque. In optical assessment, transparency determines whether an opal is classified as crystal opal (transparent where text can be read through the stone), semi-crystal opal (translucent where text appears as an unreadable shadow), or opaque opal (no light transmission).
Brightness in opals is an optical characteristic that measures the intensity of colours in the stone. The brightness scale ranges from B7 (very weak) to B1 (exceptionally brilliant), with brighter opals generally having higher value regardless of size.
Opal colour patterns are distinct arrangements of play-of-colour that form part of optical characterisation. Small-type patterns like pin-fire are less desirable than broad sweeping patterns or large flashes. Valuable patterns include straw pattern, Chinese writing pattern, ribbon pattern, Harlequin, and rolling flashes.
Some of the rarest patterns include starbursts, block, flagstone, jigsaw, and fan patterns. The pattern’s interaction with light and its distribution across the stone directly affect the opal’s value.
Spectral range is an optical characterisation subset in opal that measures how white light diffracts through the regularly arranged silica spheres in the opal’s structure. This optical phenomenon generates a range of spectral colours through light interference patterns.
Black opals are considered the rarest and most valuable of all opal varieties due to their dark body tone, which enhances the play of colour. What makes a black opal so rare is the phenomenon of a dark body tone, which is called black potch, which forms naturally on the precious opal colour to give it its bright vibrance.
