Opal is a unique gemstone classified chemically as a hardened gel of amorphous, hydrated silica, represented by the formula SiO₂·nH₂O. Unlike crystalline gems, its structure is composed of microscopic spheres of silica that contain a small amount of water, typically ranging from six to ten percent of its mass. The spectacular optical effect known as “play-of-color” is produced when light diffracts through a regular, three-dimensional arrangement of these uniform silica spheres. Australia is the world’s most significant source of this precious material, consistently supplying over 90% of the global market.
The Unique Geography of Australian Opal
The unique environment necessary for the formation of precious opal is found across the vast sedimentary basin of the Australian outback. This opal is a product of deep chemical weathering of Cretaceous-era rock, where silica-rich groundwater seeped into cracks and voids before slowly depositing the silica spheres. The resulting sedimentary opal is prized for its inherent stability, maintaining a lower and more consistent water content than opals from volcanic deposits elsewhere.
Opal production is concentrated in three main regions, each known for a distinct type of stone. Lightning Ridge in New South Wales is the primary source of the world’s most valuable black opal. South Australian fields, notably Coober Pedy, produce the bulk of lighter-toned white and crystal opals. Queensland fields, including Winton and Quilpie, are the exclusive home of boulder opal.
Classification of Australian Opal Types
Australian opals are classified primarily by their body tone, which is the underlying background color visible beneath the play-of-color. The most highly valued is Black Opal, featuring a body tone ranging from dark gray to jet black, typically rated N1 to N4 on the body tone scale. This dark background creates the greatest contrast, making the vibrant diffracted colors appear more vivid and intense. Black opal is almost exclusively found in the mines of Lightning Ridge.
White or Milk Opal is characterized by a light body tone, generally rated N7 to N9. The pale, sometimes milky, base makes the play-of-color appear more subtle, and this type is the most common and widely produced, largely originating from South Australia. Boulder Opal is identifiable because it is left naturally attached to its host rock, a dark brown ironstone matrix. This unique formation, found only in Queensland, results in a naturally backed stone that can display a very dark body tone, often cut into freeform shapes to maximize the thin veins of color.
Crystal Opal is defined by its transparency or translucency, and can occur with any body tone. Light can pass through this stone, giving the play-of-color a remarkable depth and a luminous quality. A rare Black Crystal Opal is highly sought after, combining the clarity of the crystal type with the dark body tone.
Determining Opal Quality and Value
The value of an individual opal is determined by a combination of specific aesthetic and structural criteria. Foremost is the stone’s Play-of-Color, which is judged on its brilliance, color range, and pattern. Brilliance, or brightness, is the intensity of the color flashes, where the most vivid stones command the highest prices.
The range of colors displayed is also a factor, with the presence of red being the rarest and most desirable hue. The color hierarchy is:
- Red
- Orange
- Yellow
- Green
- Blue
The pattern, or the arrangement of the color patches, contributes significantly to the stone’s worth. Extremely rare patterns like “Harlequin,” a mosaic-like pattern of large, distinct color blocks, are the most valuable, alongside “Broad Flash” and “Pinfire.”
Body tone remains a factor, with opals having a darker background (N1 to N4) typically valued higher because the contrast intensifies the visible colors. The stone’s structural integrity and cut are also assessed. This includes the thickness of the color bar, the overall shape and polish, and the absence of “dead spots” or “extinction,” which are areas lacking play-of-color. Clarity, referring to the degree of transparency and freedom from flaws like cracks or inclusions, is also important.
Basic Care and Handling
Opal is a relatively soft gemstone, ranking between 5.5 and 6.5 on the Mohs hardness scale, making it susceptible to scratching from harder materials like diamonds. Due to its composition as a hydrated silica, the stone is sensitive to extreme environmental conditions. Rapid temperature changes or prolonged exposure to intense heat can cause the stone to lose internal moisture, potentially leading to “crazing,” or fine surface cracks.
For cleaning, opal jewelry should be wiped gently with a soft cloth, warm water, and a mild, non-detergent soap. Harsh chemicals like bleach, ammonia, or abrasive cleaners must be avoided as they can damage the stone’s polish or surface. Ultrasonic cleaners are also not recommended, as their vibrations can cause internal fractures. When storing opal, opal should be kept separate from other jewelry and away from overly dry conditions, such as bank safes, which accelerate moisture loss.