Opal Gemstone Guide

What Is Opal?

Opal is a hydrated form of silica, composed of tiny silica spheres bonded with water. Unlike most gemstones, which are crystalline, opal has an amorphous internal structure that gives it unique optical properties.

Many precious opals are renowned for their distinctive play-of-colour—a mesmerising display of shifting rainbow hues that appears to dance across the stone's surface as light and viewing angle change. This optical phenomenon is what makes opal one of the most captivating gemstones in jewellery.

Types of Opal

Opals are found in several varieties, each with distinct characteristics. Not every opal displays play-of-colour, and understanding the different types helps when selecting opal jewellery.

Precious Opal

Precious opal exhibits play-of-colour, displaying flashes of spectral colours that shift as the stone moves. This is the most sought-after type of opal for fine jewellery.

Common Opal

Common opal, also called potch, does not display play-of-colour. It may be translucent or opaque and can appear in various colours, but lacks the rainbow effect of precious opal.

White Opal

White opal has a pale or white body colour with play-of-colour visible against the light background. It is one of the most common types of precious opal.

Black Opal

Black opal features a dark body tone—ranging from dark grey to black—which makes the play-of-colour appear more vivid and dramatic. Black opal is highly prized and typically commands premium prices.

Boulder Opal

Boulder opal forms within ironstone or other host rock, and the opal layer is left attached to the natural backing. This creates unique patterns and often results in striking, one-of-a-kind gemstones.

Fire Opal

Fire opal is known for its warm body colour, ranging from yellow to orange to red. Some fire opals display play-of-colour, whilst others are valued purely for their vibrant body colour.

Opal Colour and Appearance

Opal's colour range is extraordinarily diverse. Body colours can include white, grey, black, blue, green, yellow, orange, red and brown. The play-of-colour in precious opal can display virtually any spectral hue, including red, orange, yellow, green, blue, violet and combinations thereof.

Transparency varies from transparent to translucent to opaque, depending on the type and quality of the opal. The interplay between body colour, transparency and play-of-colour creates the unique character of each opal gemstone.

What Causes Opal's Play-of-Colour?

The play-of-colour in precious opal is caused by the diffraction of light through its internal structure. Opal is composed of microscopic silica spheres arranged in a regular pattern. When light enters the stone, it is diffracted by these spheres, splitting into spectral colours.

The size and arrangement of the silica spheres determine which colours are visible. Smaller spheres produce blues and violets, whilst larger spheres create reds and oranges. The regularity of the sphere arrangement affects the intensity and clarity of the colour display.

This phenomenon is what makes each opal unique—no two opals have identical internal structures, so no two opals display exactly the same pattern or combination of colours.

Opal Hardness and Durability

Opal has a Mohs hardness of approximately 5.5 to 6.5, which makes it softer and more delicate than gemstones such as sapphire (9) or ruby (9). This means opal requires more careful handling and is more susceptible to scratches, chips and fractures.

Despite its relative softness, opal can be suitable for a wide range of jewellery, including rings, earrings, pendants, necklaces and bracelets. However, opal jewellery benefits from protective settings and sensible care to minimise the risk of damage from knocks or abrasion.

Where Are Opals Found?

Opals are mined in several countries, with Australia being the most famous and prolific source. Australian opals, particularly black opals from Lightning Ridge and white opals from Coober Pedy, are highly regarded worldwide.

Ethiopia has emerged as an important source of precious opal in recent decades, producing opals with vibrant play-of-colour and a range of body tones. Mexican opals, including fire opals and crystal opals, are also well known and valued for their warm colours and transparency.

Other sources include Brazil, the United States, Peru and several African countries. Whilst geographical origin can influence market perception, the quality of an individual opal is determined by its colour, pattern, brightness, clarity and overall appearance, not solely by where it was mined.

Natural, Treated and Assembled Opals

Understanding the difference between natural, treated and assembled opals is important when purchasing opal jewellery.

Natural Opal

Natural opal is mined and cut without significant alteration to its appearance. It may be polished and shaped, but its colour and structure are as nature formed them.

Treated Opal

Some opals may be treated to enhance or alter their appearance. Treatments can include smoke treatment to darken the body tone, sugar-acid treatment to create a darker appearance, or impregnation with resins or oils to improve stability or colour. Any treatments should be disclosed by reputable jewellers.

Opal Doublets and Triplets

Opal doublets consist of a thin slice of natural opal glued to a backing material, often black potch or ironstone, to enhance the play-of-colour and provide structural support. Opal triplets add a protective clear cap, usually quartz or glass, over the opal layer.

Doublets and triplets are legitimate products and can offer beautiful, affordable opal jewellery. However, they must be accurately described and sold as assembled stones, not as solid natural opals. Reputable jewellers will always disclose the construction of opal gemstones.

Factors That Influence Opal Quality

Several factors determine the quality and value of an opal gemstone:

• Play-of-colour: The presence, intensity and range of spectral colours are the most important quality factors in precious opal. Vivid, multi-coloured displays are highly prized.
• Brightness: The brilliance and intensity of the colour flashes affect desirability. Brighter opals are generally more valuable.
• Colour pattern: The arrangement and distribution of colour—such as harlequin, pinfire, broad flash or rolling flash—can influence appeal and value.
• Body colour: The background tone of the opal affects how the play-of-colour appears. Darker body tones, such as black opal, often make colours appear more vivid.
• Transparency: Crystal opals with high transparency and strong play-of-colour are particularly valued.
• Cut and shape: A well-proportioned cut that maximises play-of-colour and minimises inclusions or flaws enhances the stone's beauty.
• Size: Larger opals with strong play-of-colour are rarer and command higher prices.
• Stability: Opals that are stable and less prone to crazing or cracking are more desirable.
• Treatments: Untreated natural opals are generally more valuable than treated or assembled stones.

Caring for Opal Jewellery

Opal contains water—typically between 3% and 21%—and can be sensitive to environmental conditions and handling. Proper care helps preserve the beauty and integrity of opal jewellery.

Avoid Extreme Conditions

Protect opal from extreme heat, sudden temperature changes and prolonged exposure to direct sunlight, as these can cause the stone to dry out, crack or craze. Remove opal jewellery before activities involving high heat or temperature fluctuations.

Protect from Impact and Chemicals

Opal's relative softness makes it vulnerable to scratches and chips. Avoid wearing opal jewellery during activities that may expose it to hard knocks or abrasive surfaces. Keep opal away from harsh chemicals, including household cleaners, perfumes and cosmetics, which can damage the stone or its setting.

Cleaning Opal

Clean opal gently with a soft, damp cloth or, where appropriate, mild soapy water and a soft brush. Rinse carefully and dry with a soft cloth. Avoid ultrasonic cleaners and steam cleaners unless a professional jeweller has confirmed they are safe for your specific opal piece, as these methods can cause damage, particularly to doublets and triplets.

Storage

Store opal jewellery separately from harder gemstones to prevent scratching. A soft pouch or lined jewellery box is ideal. Some collectors store opals in a slightly humid environment or with a damp cloth to help maintain moisture content, though this is not always necessary for stable opals.

Opal Meaning and Symbolism

Throughout history, opal has been associated with a variety of symbolic meanings and cultural beliefs. These associations are rooted in tradition and folklore, not scientific fact.

Opal has traditionally been linked to creativity, inspiration and individuality, reflecting the stone's unique and ever-changing play-of-colour. It has also been associated with hope, purity and emotional expression.

In some cultures, opal was believed to bring good fortune, whilst in others it was thought to possess protective qualities. These symbolic meanings vary widely across different traditions and time periods.

Opal as an October Birthstone

Opal is recognised as one of the birthstones for October, alongside tourmaline. This makes opal jewellery a popular and meaningful choice for those born in October, as well as for gifts celebrating October birthdays or anniversaries.

The tradition of birthstones has evolved over centuries, and opal's association with October is widely accepted in modern jewellery culture. Opal's vibrant play-of-colour and individuality make it a fitting symbol for personal expression and celebration.

Opal vs Opalite: Understanding the Difference

It is important to distinguish between natural opal and opalite, as they are entirely different materials.

Natural opal is a gemstone formed by geological processes over millions of years. It is a hydrated silica mineral with unique optical properties, particularly the play-of-colour seen in precious opal.

Opalite, on the other hand, is commonly a man-made glass or resin product designed to mimic the appearance of opal. It typically has a milky, iridescent appearance with blue or purple flashes, but it does not possess the natural structure or play-of-colour of genuine opal.

Opalite is often sold in costume jewellery and decorative items. Whilst it can be attractive, it should not be confused with or sold as natural opal. Reputable jewellers will clearly distinguish between natural opal gemstones and man-made opalite or glass imitations.