The formation of Australian opal is the result of a truly unique geological phenomenon, an ancient event that unfolded between 97 and 100 million years ago, when 60% of Australia was submerged beneath an inland sea.

As this vast sea slowly dried up, it triggered what scientists refer to as an “extraordinary episode of acidic weathering.” This rare process stretched from Coober Pedy in South Australia, through Lightning Ridge in northern New South Wales, and all the way to the Gulf of Carpentaria in northern Australia.

During this time, pyrite minerals (also known as fool’s gold) began to break down and release sulphuric acid into the surrounding environment. This caused the shallow layers of the Earth to become highly acidic, a critical step that allowed silica to be released from the sandstone rock through a long period of chemical weathering.

Over time, as the acidity levels dropped and the climate continued to evolve, a perfect window emerged for precious opal to begin forming. Silica-rich gel, carried by underground water, slowly seeped into cracks and cavities in the sandstone. This process was incredibly slow — estimated at just 1 centimeter every 5 million years.

A World-Unique Process. 

What makes Australian opals so special isn’t just their colour, it’s their origin. Central Australia is believed to be the only place on Earth where this type of acidic weathering occurred on such a vast scale. This makes Australian opals geologically and gemologically unique, which is why they are in such high global demand.

At OzOpals, we believe that truly accurate opal valuation goes far beyond a simple visual check. That’s why we’ve created the world’s first online opal pricing tool.

The OzOpal Pricing Tool is designed to provide the most comprehensive and precise opal valuation available anywhere.

This unique tool uses an advanced algorithm that draws on 16 detailed evaluation factors, refined from decades of industry knowledge and expert opal handling. Together, they form the backbone of our highly accurate online appraisal system.

The 16 Professional-Grade Opal Pricing Factors

1. Type of Opal: Black, Dark, Light / White, Semi-Crystal, and Crystal.

2. Body Tone / Base Colour: N1 (deep black) to N9 (milky white) on the official opal body tone scale.

3. Brightness: Intensity and clarity of the opal’s colour display.

4. Predominant Opal Colour: Dominant hue visible in the play-of-colour (red, red-blue multi-colour, green, blue, etc.)

5. Pattern: Type and rarity of the gem’s colour pattern (e.g., chaff, harlequin, floral, pinfire).

6. Body Tone Consistency: How even and stable the base tone appears across the stone.

7. Brightness & Colour Consistency: Uniformity of brilliance and hue distribution.

8. Colour Saturation: Depth and richness of the visible colours.

9. Colour Directionality: How visible the colours are from different angles (broad flash vs. directional flash).

10. Colour Nightshade: Whether colour remains vibrant in low-light conditions.

11. Pattern Consistency: Uniformity and balance of the visible pattern across the gem.

12. Finish – Cut: The technical quality of how the stone was cut (symmetry, accuracy).

13. Finish – Dome: Height and curve of the opal dome (key to reflection and display).

14. Finish – Shape: Final shape (standard oval, freeform etc.) and its balance.

15. Inclusions: Visible internal flaws, sand spots, potch bleed, or structural weaknesses.

16. Weight: Measured in carats, contributing to overall rarity and price per carat.

Why It Matters

Traditional valuation methods often miss subtle but crucial elements that affect the true worth of an opal. With the OzOpal Pricing Tool, you’re not guessing, you’re empowered with a highly accurate, data-driven estimate based on a complete professional-grade assessment.

Whether you’re a miner, cutter, dealer, or buyer, this tool gives you an unparalleled advantage in understanding and pricing Australian opals with confidence.

Ready to experience it?
Subscribe to the OzOpal Pricing Tool and get access to the most advanced opal valuation system ever created.

Black Opal, found in the famous Lightning Ridge opal fields of New South Wales, was first discovered in the 1880s. Today, it is recognised as the most valuable type of opal in the world, not only for its beauty but for its extraordinary rarity.

What makes Black Opal so exceptional is its dark body tone, classified on the N1 to N4 range of the Opal Body Tone Scale. This dark background dramatically enhances the gemstone’s play of colour, allowing the brilliant reds, greens, blues, and other hues to appear more vivid and vibrant than in lighter opals.

How Valuable Is Black Opal?

Gem-quality Black Opals can exceed prices of AUD $20,000 per carat, especially when showcasing strong colour brightness, rare patterns, or large carat weight. However, it’s important to understand that not all black opals are valuable. A dark body tone alone doesn’t guarantee worth, true value depends on a range of factors.

Dark Base Opal sits beautifully between Black Opal and White Opal on the Opal Body Tone Scale, with a body tone ranging from N5 to N6. These stones typically exhibit a smokey to dark grey base, providing a rich backdrop that enhances the vibrancy of their play-of-colour, though with slightly less intensity than true Black Opal.

One of the most well-known sources of Dark Base Opal is Lightning Ridge, New South Wales, where stones often bridge the classification between Black and Dark Based Opal (often referred to as “Semi-Black” Opal).

Australian Locations Known for Dark (Semi-Black) Opals

• Mintabie, South Australia
  Renowned for producing Dark Based and Dark Crystal Opals, Mintabie’s material is highly sought after by collectors and cutters alike.

• Andamooka & Coober Pedy, SA
  While primarily known for White and Crystal Opals, select fields within Andamooka and Coober Pedy also yield darker-toned opals, including Dark Base (Semi-Black) varieties. These regions have contributed significantly to the diversity of Australia’s opal landscape.

South Australia is home to some of the most productive opal fields in the world. In fact, it’s estimated that over 80% of the global supply of White Opal originates from this region.

Coober Pedy – The White Opal Capital of the World

The iconic town of Coober Pedy is internationally recognised for its abundant deposits of White and Light Opals. These stones are typically found with a body tone ranging from milky white to near-transparent, classified between N7 to N9 on the Opal Body Tone Scale.

White Opals are the most commonly found opal variety across Australian fields, but Coober Pedy stands out as their primary source. These opals often feature the full spectrum of colour displayed across an opaque or lightly translucent body.

Crystal Opal vs. White & Jelly Opal

While White and Light Opals provide a beautiful backdrop for play-of-colour, Crystal Opal is considered a superior variety due to its transparency or translucency. This allows vibrant colour flashes to appear suspended deep within the gemstone, creating a glowing, three-dimensional effect.

• Crystal Opal: Transparent to translucent, with vivid play-of-colour throughout.

• White / Light Opal: Opaque to semi-translucent, with play-of-colour across a milky or light background.

• Jelly Opal: A subtype of Crystal Opal, usually moderately coloured with a gelatinous, glassy appearance, often with less intensity than top-grade Crystal.

The Australian state of Queensland is the exclusive home of Boulder Opal, one of the most striking and structurally durable forms of natural opal. Known for its earthy body tones and vibrant colour contrasts, Boulder Opal is a geological wonder that continues to captivate collectors, lapidaries, and jewellery designers around the world.

Origin and Geology

Discovered in the 1870s, Queensland’s Boulder Opal fields now span more than 1,000 kilometres (600+ Miles) across remote regions of the state. These opals form naturally within veins and cavities of ironstone rock, and the host rock is intentionally retained during cutting and polishing, becoming part of the finished gem.

Appearance & Unique Qualities

Boulder Opals can appear light or dark in tone, and many display colour intensity comparable to Black Opal from Lightning Ridge. However, what sets Boulder Opal apart is its unique ironstone backing, which enhances the stone’s overall stability and durability.

Because of this backing, Boulder Opal is widely regarded by experts as the most stable type of natural opal on Earth. It contains very low water content, meaning it is highly resistant to cracking, crazing, or drying out over time, a somewhat common concern amongst other types of opal.

While both Black Opal and Boulder Opal are among the most vibrant and coveted types of opal, there are several key differences between them, in structure, grading, and market value.

Visual Quality & Colour Brilliance

There is little difference in the gem-grade opal quality mined from Lightning Ridge (Black Opal) and Queensland’s Boulder Opal fields. In fact, some experts argue that Boulder Opal can appear even brighter and more vibrant due to its extremely thin colour bar sitting directly against the dark ironstone backing. This natural contrast amplifies the brilliance of the opal’s play-of-colour.

Both Black and Boulder Opals display the full rainbow spectrum, often showing the same prized opal patterns such as:

• Floral

• Harlequin

• Ribbon

• Straw

• Chinese Writing (Script)

• Rolling Flash

Price & Affordability

Despite their visual similarities, Boulder Opal is generally more affordable, often priced at around one-third of the cost per carat compared to Black Opal of similar quality. This makes Boulder Opal an excellent choice for those wanting high-colour brilliance at a more accessible price point.

Grading Differences

One notable distinction lies in how these opals are graded and classified:

• Black Opal is graded using the official Scale of Body Tone (N1–N9), where N1–N4 represent true black opals.

• Boulder Opal, on the other hand, naturally includes its ironstone backing, so body tone is not measured in the same standalone way. The darkness of the backing contributes inherently to its overall look.

The way an opal is cut plays a major role in how it’s valued and appreciated in jewellery design. Traditional cutting styles differ across White Opal, Black Opal, and Boulder Opal, largely due to how each stone forms in nature.

Cabochons and Symmetry – Traditional Cuts

White and Black Opals are typically cut into symmetrical, domed shapes, such as ovals, rounds, and teardrops known in the industry as cabochons. These shapes are highly desirable in global jewellery markets due to their balanced proportions, polishability, and ease of setting.

Modern Trends – Free-Form Opal Shapes

However, modern jewellery design is evolving. The industry is seeing a growing demand for free-form (baroque) shaped opals, thanks to advances in design techniques and a shift toward one-of-a-kind pieces. These organic, asymmetrical cuts allow the natural form of the opal to shine, offering unique character and creative versatility.

Boulder Opal – Naturally Irregular Beauty

Unlike Black or White Opal, Boulder Opal forms in thin, undulating veins within solid ironstone rock. This natural formation makes it nearly impossible to cut traditional domed cabochons without significant material loss.

To preserve both the carat weight and the stunning natural colour bar, Boulder Opals are usually cut into free-form or irregular shapes. These unique silhouettes appeal to buyers who are drawn to earthy aesthetics, asymmetrical jewellery, and a more natural, organic feel.

Opal is a relatively soft gemstone compared to others, and its durability is an important factor to consider when wearing or setting it in jewellery.

On the Mohs scale of mineral hardness, Black, White, and Boulder Opals all fall within the range of 5.5 to 6.5. For comparison:

• Tanzanite is rated at 6.5.

• Quartz is around 7.

• Diamonds are a 10, the hardest naturally occurring mineral.

This means that opals are similar in hardness to glass and should be worn with care to avoid scratches or knocks.

Why Boulder Opal Has an Edge

While all opal types have similar hardness levels, Boulder Opal is naturally reinforced by its ironstone backing, the host rock in which it forms. This attached ironstone layer adds structural strength and stability, giving Boulder Opal a clear durability advantage over other opal types.

As a result, Boulder Opals are far more resistant to cracking or crazing from accidental impact or sudden changes in environment, making them a popular choice for everyday wear and more rugged jewellery settings.

Brightness is one of the most important and defining characteristics of precious opal. It describes how vivid and intense the play-of-colour appears under different lighting conditions, from direct sunlight to low light, and even shade.

OzOpals uses a refined B1–B5 Brightness Grading Scale as part of our OzOpal Pricing Tool, providing a highly accurate assessment that few in the industry offer.

The OzOpal Pricing Tool Brightness Scale

Each level on the brightness scale is graded based on how the opal performs in various lighting environments. Here’s how the scale works:

B1 – Faint Brightness

Colour only appears under direct sunlight, and even then, it is very faint and difficult to detect.

B2 – Dim Brightness

Displays a slight hint of colour under low light. Under indirect sunlight or a grading lamp, the colour remains muted and dim.

B2.5 – Somewhat Bright

Shows a modest improvement under both low light and indirect sunlight. These opals have more life than the Faint or Dim grades.

B3 – Bright

Displays noticeable colour under low light and very good colour under indirect sunlight or a grading lamp. A significant step up in visibility.

B3.5 – Bright +1

Shows strong colour under low light, and vivid colour under indirect sunlight or grading lamp condition. Often considered lively and eye-catching.

B4 – Very Bright

Reveals crisp, vivid colour in most lighting conditions, with excellent performance under grading lamps and indirect sun.

B4.5 – Extremely Bright

Colour is exceptionally vivid under all lighting, including low light. Under indirect sunlight, the colour appears striking and luminous.

B5 – Brilliant

This is the highest level of brightness. These opals exhibit mirror-like colour that shines vividly in low light, indirect sunlight, and even in shade or nighttime (nightshade) conditions. Rare, breathtaking, and highly valuable, these Opals tend to ‘glow’ in your hand.

Doublet Opals, often referred to as “Doublets” in the industry, are man-made opals crafted by combining two distinct layers. These opals are designed to imitate the appearance of natural Black Opals, offering a visually stunning and more affordable alternative.

How Doublets Are Made

A Doublet consists of:

• Top Layer: A thin slice of natural gem-quality Crystal or White Opal, cut and shaped into a cabochon (domed) surface to resemble a full-bodied opal.

• Backing Layer: A dark material bonded to the back of the opal slice using epoxy resin, often tinted with black chalk powder or black oxide to enhance the stone’s colour intensity.

Common backing materials include:

• Black potch (common opal).

• Dark boulder opal ironstone. 

• Industrial black glass. 

• Hardened black plastic. 

This backing mimics the dark body tone of a true Black Opal, making the colours in the opal layer appear brighter and more vivid, just like in N1–N4 “Black Opals”.

How to Identify a Doublet

Doublets can often be identified by viewing the stone from the side. A perfectly straight, horizontal line is typically visible where the two layers are bonded, a telltale sign of its constructed nature.

However, once set in jewellery with the sides covered (such as bezel settings), Doublets become extremely difficult to distinguish from solid opals, even to experienced gemologists, because the top layer is made from real opal.

Why Choose a Doublet?

• Offers the look of a Black Opal at a fraction of the price. 

• Maintains a natural opal appearance, unlike triplets.

• Ideal for jewellery, especially when set in protective settings.

Doublet Opals are highly sought after in the market for their affordability and their impressive visual similarity to genuine solid opals.

Triplet Opals, or “Triplets” as they are known in the opal industry, are three-layer assembled stones designed to replicate the appearance of high-quality Black Opals. While they contain a thin layer of natural opal, the rest of the structure is composed of man-made materials, making them an affordable and durable entry point for opal enthusiasts.

How Triplet Opals Are Constructed

Triplets consist of three distinct layers bonded together with clear epoxy resin:

1. Backing: A dark base, similar to that used in Doublets (e.g., black potch, ironstone, glass, or hardened plastic).

2. Centre: A paper-thin slice of natural Crystal or White Opal, generally much thinner than what’s used in Doublets.

3. Top Cap: A clear domed covering, typically made from glass, quartz, or sometimes plastic. This protective cap magnifies the colour beneath and shields the fragile opal layer from damage.

How to Identify a Triplet

Triplet Opals are typically easy to spot, especially to trained eyes, due to their glassy, reflective surface and enhanced shine from the artificial top cap.

Key visual indicators include:

• A perfectly flat and visible horizontal line where all three layers meet.

• A distinct “see-through” effect caused by the clear cap.

• A slightly magnified colour appearance due to the optical dome.

Why Choose a Triplet Opal?

• Very affordable: Contains the least amount of real opal, making it budget-friendly.

• Visually striking: The clear top enhances colour brightness.

• Durability: The synthetic cap makes it more impact-resistant than solid opals or doublets.

However, because the actual opal content is minimal, triplets are considered decorative rather than collectable, and are typically not valued as high-end gems.

Pro Tip:

While Triplet Opals are ideal for costume jewellery, they should be protected from prolonged exposure to water or extreme heat, as this may weaken the epoxy over time.