Opal Treasures: A Journey Through the Spectrum of Gemstone Types

Opal is a stunningly beautiful gemstone that has captivated people for centuries with its play of color and iridescent flashes. This amorphous mineraloid is highly sought after in the form of precious opal, which exhibits an array of spectral colors. However, there are actually several different types of opal, each with their own distinctive characteristics.

The most common type is known as common opal. As the name suggests, this variety is more abundant and does not display the coveted play-of-color. Common opal can be opaque or translucent and comes in various colors like white, yellow, brown, gray, and blue. It is still used for jewelry, but is less valuable than precious opal.

Precious opal is the most prized type and includes categories like black opal, crystal opal, boulder opal, and fire opal. Black opal, found in Australia, is characterized by its dark body tone that makes the color flashes pop. Crystal opal is transparent to translucent with a crystalline structure. Boulder opal forms within ironstone boulders and often has a natural, rugged shape. Fire opal from Mexico displays vibrant orange and red colors that seem to flicker like flames.

The diversity of opal types provides a range of beauty and value for collectors and jewelry designers. Each formation process and original environment lends unique qualities to the final gem. By understanding the distinctions between common, precious, and fire opals, one can better appreciate the complexity of this mesmerizing mineraloid.

What is Opal?

Opal is scientifically classified as a mineraloid due to its amorphous, non-crystalline structure. Rather than orderly aligned crystals, opal consists of silica spheres and water that hardens into solid form. The water content can range from 3% to 21% and becomes suspended within the rigid silica framework.

There are two overarching categories of opal:

• Common opal – This refers to varieties that do not display “play-of-color,” the shimmering rainbow effect for which opals are prized. Common opal exhibits a more uniform coloration and body tone.
• Precious opal – Only about 5% of opal falls into this rare category that exhibits brilliant play-of-color. Precious opal can come in several vivid varieties.

Now let’s explore some of the most popular kinds of opal in more detail.

Main Types of Opal

Black Opal

Hailing from Australia, black opal is characterized by a dark body tone underlying the play-of-color flashes. Commonly dark gray to black, this background allows the kaleidoscope of colors to really pop in contrast. Black opal is generally the most expensive and sought-after variety.

Within black opal, there are levels of darkness described as:

• Semi black – Medium gray background color
• Black – True black or very dark gray tone
• Super black – Jet black background that makes colors jump out

Black opal can display every color of the spectrum when turned in the light. Red and green flashes tend to be the most rare and valuable. Black opal is found in Lightning Ridge and other parts of New South Wales, Australia.

White Opal

Also called “milky opal,” this variety has a light body tone that makes the play-of-color stand out beautifully. White opal can range from translucent to opaque in clarity. It comes from Australia as well as other regions like Brazil and parts of the United States.

White opal shows off the full range of spectral colors. Valuable specimens exhibit bright patches of red, orange, green or blue fire. The most prized white opal has a bluish hue similar to the body tone of a fine black opal. White opal is occasionally treated to darken or blacken the background to resemble black opal.

Crystal Opal

As the name suggests, crystal opal has a crystalline structure made of dense silica spheres. This makes it transparent to translucent, unlike other opal types. Due to the orderly silica formation, the play-of-color takes on a unique striated or spiral pattern rather than blotchy patches.

Crystal opal is found in Australia as well as Mexico. It can display a range of body tones, with darker backgrounds being most rare and desirable to accentuate the play-of-color.

Boulder Opal

Boulder opal forms within natural cavities and cracks in ironstone boulders. Valuable specimens of boulder opal come from Queensland, Australia where iron-rich sedimentary rocks are common. Boulder opal has an organic, asymmetric shape that reflects the shape of the original stone matrix.

Boulder opal usually requires cutting and polishing the rough, irregular stones to reveal the opal layers and vivid play-of-color within. The surrounding matrix rock increases the density and makes boulder opal ideal for jewelry. Fine quality boulder opal exhibits colors like red, green, and blue.

Fire Opal

As its name suggests, fire opal blazes with fiery flashes of orange and red – the most scarce colors among precious opals. It really does appear to flicker and glow like a burning ember. Most fire opal comes from Mexico, particularly in the Mexican states of Querétaro and Nayarit. Fire opal is also found in Oregon and Nevada in the United States.

Fire opal is transparent to translucent and can be faceted into brilliant cuts that maximize its intense colors. Stones exhibiting bright reds and oranges set against a lighter background are most desirable.

Matrix Opal

Matrix opal refers to precious opal stones still enclosed within their original surrounding host rock. The matrix rock, often ironstone or sandstone, displays seams or concentrations of vivid opal running through it.

Like boulder opal, matrix opal is cut and polished to expose the opal layers. Australia produces most of the world’s matrix opal, notably Koroit opal from South Australia which displays beautiful play-of-color in ironstone.

Hydrophane Opal

This rare variety exhibits an incredible sponge-like ability to absorb water, causing the material to noticeably change transparency. When dry, hydrophane opal appears opaque. But when immersed in water, it turns translucent and reveals new color flashes.

Hydrophane comes almost exclusively from Indonesia and requires care to prevent damage when wet. It has a porous, lightweight structure unlike common opal.

Pipe Opal

As the name implies, pipe opal forms in long, cylindrical hollows resembling pipes within a host rock like claystone or ironstone. Pipe opal is found in Australia. The opal typically lines the inside walls of these unique tubular formations, requiring the surrounding rock to be cut away.

Comparison of the Major Types of Opal

Type of Opal	Origin/Sources	Colors	Background Color	Pattern	Clarity	Value
Black Opal	Australia (New South Wales)	All colors, especially red, green, blue	Darkest gray to jet black	Patchy, spotty, irregular bands	Translucent to opaque	Most expensive and valuable variety
White Opal	Australia, Brazil, Honduras	All colors, especially red, orange, green	Whitish, cream, pale gray	Patchy spots	Translucent to opaque	Affordable, moderately valuable
Crystal Opal	Australia, Mexico	All colors	Transparent to pale gray	Striated, banded	Transparent to translucent	Highly valuable for its clarity
Boulder Opal	Australia (Queensland)	Red, green, blue	Natural ironstone matrix	Irregular patches, seams	Opaque	Valuable, bold pattern
Fire Opal	Mexico, USA (Oregon, Nevada)	Reds, oranges, yellows	Pale to medium orange	Spotty, blotchy	Transparent to translucent	Very valuable, vivid colors
Matrix Opal	Australia	Rainbow	Natural host rock like ironstone	Veins, seams	Opaque	Moderately valuable
Hydrophane Opal	Indonesia	Pale colors	Varies, opaque when dry	Irregular patches	Changes transparency with water	Highly rare and valuable
Pipe Opal	Australia	Rainbow	Walls of pipe cavities	Hollow tubular bands	Opaque to translucent	Unique form increases value

Opal Doublets and Triplets

In order to make opal more affordable and durable for use in jewelry, some stones are assembled into doublets or triplets.

• Doublets consist of a thin slice of precious opal adhered to a dark backing material like obsidian, ironstone, or basalt.
• Triplets take it a step further by adding a clear domed cap of quartz or glass on top of the opal layer.

Creating doublets and triplets allows the mesmerizing play-of-color to be seen in pieces with very thin layers of opal that would otherwise be too fragile on their own. However, these manufactured composites are worth significantly less than solid natural opal gems. They can also sometimes come apart if the glues weaken over time.

Opal Background Colors

The background color visible behind the play-of-color in a precious opal has a major impact on its appearance and value. Darker body tones provide more contrast against the kaleidoscopic flashes. Here are some of the most important background shades:

• Black – The rarest and most valuable background, makes colors really pop.
• Dark gray to blue – Increased value over light opals. Allows bright play-of-color to stand out.
• Light gray to white – The most common and affordable body tones. Still vividly colored.
• Crystal or transparent – Unique look, rarer than white opals.
• Boulder – Natural ironstone matrix in various dark tones.

Opal Cutting Styles

Opals are cut into a variety of shapes designed maximize their play-of-color. The most common cuts are:

Cabochon

Smoothly domed top with a flat base. This traditional style works well to display opal’s colors and iridescence.

Freeform

Asymmetrical and organic shapes following the shape of the raw opal stone. Favored for boulder opal.

Oval

Gently rounded oval shape. An alternative to round brilliant cuts. Minimizes waste when cutting oval opal rough.

Round

A classic cut that displays opal’s rainbow patterns in all directions. The domed or spherical top adds brilliance.

Square/Rectangular

Offer straight edges and large flat surfaces to showcase opal’s color patches. Common for matrix opal in slabs.

Opal Sizes, Weights, and Shapes

In jewelry, opals typically range in size from about 0.1 carats to 10 carats or more. The ideal weight depends on the setting style and design.

Some famous giant opals include:

• The “Olympic Australis” – 17,000 carats (3.4 kg)
• “The Andamooka Desert Flame” – 6,100 carats (1.2 kg)
• “The Virgin Rainbow” – 11,000 carats (2.2 kg)

Due to its amorphous structure, opal has a lower density than crystalline gemstones. An opal cabochon will appear larger in carat weight compared to a diamond of the same carat weight. This can be advantageous when using opal as a center stone to maximize visual impact. However, opal’s relatively low hardness of 5.5 to 6.5 on the Mohs scale means large opals require protective settings.

Identifying Opal Treatments

Some opals are altered or treated to improve their appearance and durability. Here are some things to look out for:

• Doublets or triplets – Examine stone for visible glue lines separating layers
• Impregnation – Treatment with oils, waxes, or resins to fill cracks. Dulls play-of-color.
• Dyeing – Unnaturally vivid patches of color. Color concentration is often uneven.
• Heat treatment – Removes water content but causes fracturing and reduces play-of-color.
• Coating – Thin surface coating to improve color. Wears off over time.

Always buy opal from trusted dealers who fully disclose any treatments. Genuine untreated opal in good condition will command higher prices.

Unique Opal Sources

While Australia is the dominant producer of precious opal, other regions offer rare varieties:

• Mexico – Valued source of vibrant fire opal
• Brazil – Fine crystal opal and bright white opal
• Indonesia – Nearly exclusive source of stunning hydrophane opal
• Oregon – American fire opal from Opal Butte
• Nevada – Variety of opal types like fire and crystal opal
• Ethiopia – Impressive black opal distinct from Australia’s

Caring for Opal Jewelry

Due to its delicate nature, opal jewelry requires special care:

• Avoid pressure, blows, or abrasion that could fracture opal.
• Clean only with warm water and mild detergent. Ultrasonic and steam cleaning may damage opal.
• Keep opals away from chemicals, heat, and prolonged sunlight exposure.
• Store opal carefully in soft cloth or alone in a compartment to prevent scratches.
• Consider removing opal rings, bracelets, and earrings for physical activities, washing dishes, etc.
• Have opal checked annually by a jeweler for cracks or loose settings.

Frequently Asked Questions about Opal Types

What is the most expensive and valuable type of opal?

Black opal is generally the most expensive and coveted variety due to its dark body tone that allows the play-of-color to really stand out. Fine quality black opals with strong red or green flashes command the highest prices.

What is the difference between precious opal and common opal?

Precious opal exhibits the brilliant play-of-color for which opals are prized, while common opal does not display this phenomenon and has a more uniform, mundane appearance. Only about 5% of all natural opal is precious opal.

What causes the play-of-color in precious opals?

This iridescent effect is caused by the diffraction and interference of light passing through the ordered silica spheres that make up opal’s structure. Small variations in the silica layers refract light into different colors.

Where does boulder opal come from?

Boulder opal is mined in Queensland, Australia where it naturally forms within cavities and fractures in ironstone boulders. The boulder matrix gives boulder opal a very rugged and organic appearance.

What is fire opal?

Fire opal is a transparent to translucent opal with warm body tones of orange and red. It exhibits fiery flashes of color due to its high iron content. Fire opal primarily comes from Mexico.

What is crystal opal?

This is a transparent variety of opal with an orderly silica structure that allows light to pass through. Crystal opal has a crystalline appearance and often displays striated or spiral colored patterns.

Can opal be treated or enhanced?

Yes, some methods used to improve opal include dyeing, heating/dehydrating, impregnation with oils/waxes, and assembling doublets/triplets by combining thin layers with other materials. Always buy from reputable sellers.

How fragile is opal compared to other gems?

With a hardness of only 5.5-6.5, opal is relatively soft and fragile compared to crystals like diamonds, sapphires, and rubies which score over 9 on the Mohs hardness scale. Opal requires protective settings and gentle handling.

Conclusion

In conclusion, the types of opals are as diverse as it is captivating, offering a kaleidoscope of colors and characteristics that have fascinated gemstone enthusiasts for centuries. Throughout our exploration, we’ve delved into the many types of opal, from the fiery brilliance of precious opals to the soothing blues of common opals and the mesmerizing play of colors in black opals.