Opals hold a special status as Australia's national gemstone, and are prized for their exceptional value and beauty in both gems and jewelry. What sets them apart is a unique optical phenomenon that they exhibit, known as Opalescence or “Play of Color”. This results in a shimmering light of rainbow colors that seem to shift and float on the surface of opal stones, and result from the unique structure and chemical composition of opals, which diffract and reflect light. This play-of-color, combined with diverse body tones, colors, and patterns, gives rise to many exquisite opal varieties. In astrology, opals are the birthstone for Libra and are greatly recommended by astrologers to strengthen Venus, the planet of love, luxury, and beauty.
In this article, we survey the major types of natural opal stones and what distinguishes them. In a general sense, opals fall into 2 overarching categories - precious opal and common opal. Precious opal exhibits play-of-color from the internal diffraction of light off ordered silica particles. These rare opals are the most valuable type. Common opals do not display play-of-color, but they have their visual appeal. Specific varieties exist within each group based on body color, carat weights, transparency, inclusions, place of origin and more. Keep reading to understand everything you need to know about the various types of opal stones.
What Are Opals?
Unlike most precious and semi-precious gemstones such as quartz, sapphires, emeralds, rubies, etc. which are crystalline minerals, natural opals are an amorphous variety of gemstones that are classified as mineraloids. They are composed of silicon dioxide and water, with variable water content between 3-21% by weight. Mineraloids share some similarities with minerals, but they lack the defined crystalline structure and ordered molecular framework found in true minerals.
The random arrangement of silica spheres in an opal, combined with their unique amorphous structure creates a magnificent optical phenomenon called "play of color" which is the trademark of precious opals. Play-of-color refers to flashes of spectral color - red, green, blue, yellow, orange, violet - that flare across the opal as it is turned. This shimmering rainbow effect results from the way light is diffracted by the internal structure of the opal. Play-of-color is what makes opals so magical and desirable. The intensity and quality of color display have a huge impact on the value of the gem.
Rough opal stones
No other gemstone produces this type of diffractive photometric effect. As silica spheres become less organized, play-of-color fades and becomes chaotic. Common opals lack organized silica spheres to exhibit brilliant flashes. But precious opals with strong play-of-color command astronomical prices that rival other precious gems.
Types of Opals
Broadly speaking, original opal gemstones fall into 2 main categories – depending on whether or not they exhibit opalescence/play of color.
- Precious Opal: Exhibits play-of-color from ordered silica spheres diffracting light. The intensity of the play of color varies from gem to gem. Precious opals are primarily found in vein fillings and fractures in the rock, and Australia is considered to be the most important source of precious opals.
- Common Opal: Common opals generally do not exhibit a play of color, and if they do – it is usually very faint. These common opals are also more abundant than precious opal stones, and are consequently less valuable. Common opals include hyalite, hydrophone, fire opal, and boulder opal (depending on the type, boulder opal may exhibit play-of-color).
While common opal gemstones have their own beauty and desirability, precious opals with vibrant flashes of spectral color are rarer and more valuable. Precious opals are further subdivided into even more specific varieties based on their color patterns, carats, and origins. Below we explore the granular classifications of the different types of opals available in the market today.
Types of Precious Opals
- Black Opal: Black opal is the most prized and valuable variety of opal stones. It originates primarily from Lightning Ridge in New South Wales, Australia. Contrary to a common misconception, the descriptor "black" does not indicate the opal is completely black in color. Rather, it means the body tone is darker relative to white opals. The rich, black background provides a contrast that makes flashes of internal play-of-color stand out beautifully. High-quality black Australian opals with a vibrant display of color are extremely rare and valuable. Black opals are also found in Ethiopia – but are considered somewhat inferior to those found in Australia.
Rough black opals (Source: GIA)
- White Opal: White opals have a light body tone that ranges from white to medium grey. They are sometimes referred to as "white opal", though this term is best used only when the body color is cloudy or milky. Compared to the dark backdrop of black opals that make play-of-color pop, the paler body of white opals generally doesn't highlight their color flashes as intensely. However, white opals tend to be more common like garnet. Vibrant high-quality specimens can still display stunning patches of spectral fire that rival black opals. While generally more affordable, exceptional crystal clarity or color in white opals makes them valuable in jewelry or as collector's pieces. Although white opals occur in all carat weights, they are usually found in larger pieces with heavier carat weights – usually 8 carats or above.
White opal with play of color
Types of Common Opals
While common opals do not display play-of-color, they have their striking properties. Varieties include:
- Fire Opal: Fire opal stone is a transparent to translucent gemstone displaying warm hues of yellow, orange or red as its body color. While it typically does not exhibit play-of-color, some fire opals show bright flashes of green. Fire opal is not a term used often in the Australian opal industry. However, the Mexican state of Querétaro produces the most famous fire opals, referred to as Mexican fire opals. Mexican opals that lack play-of-color are sometimes called jelly opals. Occasionally, Mexican fire opals are cut with the surrounding rhyolite host rock intact. These are known as Cantera opals. Fire opals are also found in Ethiopia, albeit of somewhat inferior qualities.
A Mexican Fire Opal
- Boulder Opal: Boulder opal is a type of opal gemstone where the surrounding host rock, usually ironstone, forms naturally as part of the gem. Boulder opals are primarily found in Western Queensland, Australia, where they occur in thin seams filling cracks and gaps in ironstone boulders. Viewed from above, this Australian opal can appear either black or light-toned depending on the body color of the thin opal layer. A distinctive feature of boulder opal is its tendency to cleave along the opal vein, leaving two naturally polished opal surfaces where the stone splits. The ironstone host rock makes boulder opal stone quite durable and provides a striking visual contrast against the fiery play of color.
Boulder Opal with prominent Opalescence
- Matrix Opal: Matrix opal refers to opal stone material that is diffused into the pore spaces within its host rock. The term is used when the opal is closely intergrown with the host rock grains. Ironstone is a common host rock within which matrix opals are formed – found primarily in Queensland, Australia. Andamooka matrix opal is a porous opal material from Andamooka, South Australia. This natural opal has a dark appearance due to carbon deposited in pores and openings. The opal's porous structure and heavy carats also allow it to be artificially enhanced through a process involving soaking in sugar solution and acid. This treatment precipitates additional carbon onto the available internal surface spaces, amplifying its dark tone.
A Matrix Opal
- Water Opal: Water opals and jelly opals are transparent to translucent opals with a gelatinous appearance. Their thickness and high water content produce an effect resembling flowing water, hence the name. Most water opals originate from Mexico or Ethiopia. While they lack play-of-color, their appearance is unique.
A Water Opal
- Hyalite Opal: Hyalite, also known as Muller's Glass, is a clear, colorless form of opal with a vitreous luster resembling glass. Though typically transparent, hyalite sometimes displays a faint blue, green or yellow tinge. The two most famous hyalite localities are Oregon and Mexico. A special type of hyalite from Mexico is girasol opal. It exhibits an unusual optical effect where a bluish glow moves across the opal as the light source is repositioned. Its beauty stems from the pure colorless body allowing the glowing phenomenon to be readily observed.
A Hyalite Opal
- Hydrophane Opal: Hydrophane opals absorb water, becoming transparent when wet with colors muted. This effect results from their higher porosity and ability to take up water into pore spaces and fractures within the mineraloid structure. When dry, hydrophane opals appear opaque to translucent. But when immersed in water, they become largely transparent as light can now travel through the water-filled internal cracks. This transparency also causes any color to appear much paler and fade when wet. These unique optical properties - the transformation from opaque to clear and muting of colors when hydrated - are what defines hydrophane opals. Due to their porous structure, their carat weights may vary depending on the amount of moisture or water absorbed by the stone. This distinguishes them from other opals that show play-of-color through light diffracting internally off their internal inclusions. Their unusual water-dependent appearance still makes them desirable for birthstone jewellery, and the most common sources are Mexico and Western Australia.
- Peruvian Opal: Peruvian opal, also referred to as blue opal, is a cloudy blue-green common opal cabochon sourced from Peru. The semi-opaque to opaque cabochon opals are frequently fashioned to showcase the surrounding host rock matrix in cut gems. While Oregon and Nevada also produce blue opal, the Peruvian variety is distinct and instantly recognizable by its vivid bluish-green coloration and heavier carat weights.
Peruvian Opals
Conclusive Reflections
Opals have captivated minds in the gem world for ages and for good reason – their optical phenomena, rarity, and astrological significance have all added to the allure of these cabochon gemstones. Ancient Greeks believed them to be Zeus’s tears, while modern astrologers believe that they are intrinsically connected to the planet Venus. Regardless of this, however, what one cannot ignore is the sheer beauty and rarity of these lab-certified gemstones.
Whether you’re looking to buy opal online for jewellery or astrological purposes, we invite you to explore Precious Earth’s collection of these mesmerizing gemstones – where each individual gem comes with its own lab-certified guarantee of authenticity and a lifetime returns policy, so that you can rest assured of a genuine purchase at our web store.
