Last updated on October 30, 2023
Opals are unique gemstones that display dazzling play of colour mixed with brightness, shimmer and sparkle, which is why these semi-precious gems have adorned men since the dawn of history. Opal, one of the birthstones of October, is regarded as one of the most beautiful stones and must-have gemstones for many jewellery lovers.
Opal Origin and Formation
The name “opal” finds its roots in the Greek word “opallios” and the Roman word “opalus,” both of which trace their origin to the Sanskrit word “upala,” meaning “precious stone.”
The formation of opals is a complex geological process that occurs over millions of years. Opals begin to form when silica-rich water, carrying dissolved silica (silicon dioxide), seeps into fissures and cavities in the host rock. This can happen in a variety of geological environments, including sedimentary rocks, volcanic rocks, and even in the roots of trees.
Over time, the water evaporates or moves away, leaving behind a silica-rich gel-like substance. This gel hardens and forms a structure that contains countless tiny spheres of silica, which are stacked closely together. The brilliant play-of-colour in opals is a result of the interference and diffraction of light as it passes through the ordered arrangement of these silica spheres. The size and uniformity of these spheres, as well as the spaces between them, determine the specific colours and patterns that will be visible in the opal.
Opals are found in various locations around the world, but the most renowned sources include Australia, Ethiopia, Mexico, Brazil and the United States. Opals can also be found in places like Honduras and Peru; however, Australia continues to stand as the primary source of the finest opals, accounting for roughly 95% of all mined gems.
Opals can display a wide spectrum of colours, often including hues from the entire rainbow. Common colours seen in opals include red, orange, yellow, green, blue, violet, and a variety of intermediate shades. Opals can exhibit single colours or a combination of multiple colours in intricate patterns.
The most distinctive feature of opals is their play-of-colour. This phenomenon is characterized by vibrant and dynamic flashes of colour that appear to dance across the surface of the gemstone as it is viewed from different angles and under various light sources. The play-of-colour is created by the interference and diffraction of light within the opal’s internal structure.
Some opals may have a dominant colour or set of colours that are more pronounced and recurrent in their play-of-colour. For instance, an opal might predominantly display shades of red, making it a “red opal,” even though it may also have other colours in the play-of-colour.
The play-of-colour in opals can manifest in various patterns, including pinfire, harlequin, rolling flash, broadflash, and more. These patterns can add a distinct visual appeal to the gemstone, enhancing its uniqueness and beauty.
In addition to the play-of-colour, opals have a body colour, which is the predominant hue of the gem or the background colour. The body colour can range from transparent and colourless to various shades of white, grey, black, and even other colours like yellow, orange, and red. For example, a “black opal” often features a dark body colour with intense play-of-colour, while a “white opal” typically has a light or white body colour. The body colour can greatly influence the overall appearance of the opal.
The rarity and desirability of opals can be influenced by the rarity of certain colours. Red is often considered the rarest and most valuable colour in opals, especially when found in combination with a dark body colour. Opals that feature red as the predominant colour are highly prized by collectors and connoisseurs.
Opal clarity refers to the transparency and visual purity of an opal, particularly in terms of its body colour. While inclusions can affect the clarity of the opal, they are not necessarily considered undesirable in all cases. In fact, some inclusions, such as matrix or dendritic patterns, can add character and uniqueness to the gemstone.
Opals can range from transparent to translucent and opaque. Transparent opals allow light to pass through them with minimal obstruction, providing a clear view of their body colour and play-of-colour. Transparent opals are highly prized for their purity, as they allow for the most vivid and brilliant play-of-colour to be observed.
Translucent opals have some degree of cloudiness or haziness in their appearance, but they still transmit light. This moderate level of transparency can create a softer and more diffused play-of-colour, giving opals a mystical and dreamy quality.
Opaque opals do not transmit light, rendering them almost entirely non-transparent. They have a solid, non-see-through appearance and often lack a distinct play-of-colour. While opaque opals may not have the vibrant play-of-colour found in transparent or translucent opals, they can still possess captivating body colours and patterns.
Some opals, known as hydrophane opals, have the remarkable property of absorbing water. When soaked in water, they temporarily enhance their clarity and become more transparent, making their play-of-colour more vivid and apparent.
The cut quality of an opal, also known as the opal’s “make,” refers to the way the gem has been shaped, faceted, or polished. While opals are not typically cut with facets like diamonds and other gemstones, their cut quality is still an important factor in determining the overall appearance and value of the opal.
Opals are typically cut into cabochons, which are rounded and polished with a smooth, domed top and a flat or slightly rounded bottom. Faceting opals is rare and challenging because the stone is not suited for traditional faceting. Faceted opals are more of an exception than the norm.
The thickness of the opal can significantly impact its durability and appearance. Opals should be cut to an appropriate thickness to ensure they are robust enough for use in jewellery. Thick opals can be carved into cabochons, which provide stability and a smooth surface for play-of-colour.
Opals have a play-of-colour that depends on the orientation of the stone. Gem cutters aim to maximize the play-of-colour by orienting the opal so that it can be viewed at its best. The surface of the opal should be expertly polished to enhance its overall appearance. A well-polished opal will have a smooth, lustrous surface that allows light to interact with the play-of-colour to its fullest potential.
Opal Carat Weight
Carat weight, like in other gemstones, significantly influences the rarity and value of opals. Large opals with high-carat weights are rarer than smaller ones. While opals of various sizes exist, those in the upper range of carat weights are less common in nature. This rarity stems from the geological conditions required for opal formation, including the presence of silica-rich solutions and the time required for opals to develop.
Large opals with impressive play-o-colour are highly prized for their rarity, visual appeal, and value per carat. However, opal enthusiasts and jewellery designers have the flexibility to choose opals of varying carat weights based on their budgets and artistic preferences.
|Chemical Name||Hydrous silicon dioxide|
|Colours||Colourless, white, yellow, orange, pink, red, green, blue, brown and black|
|Hardness||5.5 - 6 on the Mohs scale|
|Refractive Index||1.37 - 1.52|
|Specific Gravity||1.9 - 2.5|
|Transparency||Transparent, translucent and opaque|
|Fluorescence||White opal - white, bluish, brownish, greenish; fire opal - greenish to brown|
Featured image: Alf Manciagli / Shutterstock