What Is Opal?

What is opal? Opal is a hydrated form of silica known for an extraordinary optical phenomenon called play-of-color, where flashes of different colors appear to move across the stone as it is viewed from different angles. Some opals can reveal blue and green flashes, while exceptional specimens may display yellow, orange, red, violet, and multiple colors simultaneously.

Yet not every opal shows play-of-color. Opal is a surprisingly diverse gem material, ranging from transparent and translucent stones to opaque white, gray, orange, brown, and nearly black varieties. Understanding why opals can look so different begins with their unusual structure and the way they form.

How Does Opal Form?

Opal commonly forms when silica-rich water moves through cracks, cavities, and spaces within rocks. As water gradually evaporates or conditions change, microscopic silica particles can accumulate and eventually form opal.

Unlike crystalline quartz, opal does not have a conventional repeating crystal structure. Gemologists therefore classify it as a mineraloid rather than a true crystalline mineral.

Opal also contains water within its structure. The amount varies between specimens, but this characteristic contributes to some of the special care considerations associated with opal jewelry.

Different geological environments can produce remarkably different material. Opal may occur within volcanic rocks, sedimentary deposits, and cavities or fractures in host rock, depending on the locality.

Why Does Opal Show Different Colors?

One of opal's most fascinating characteristics is play-of-color.

In precious opal, microscopic silica spheres can be arranged in sufficiently ordered patterns. Light interacting with this structure is diffracted and separated into spectral colors. The particular colors that become visible depend partly on the size and arrangement of the spheres and the viewing angle.

This is why an opal can suddenly flash green, blue, orange, or red as the stone or light source moves.

The phenomenon is structural rather than simply pigmentation within the gemstone. That distinction is important: the rainbow-like colors of precious opal are produced by the interaction between light and the stone's internal structure.

Precious Opal vs. Common Opal

One of the most useful distinctions for anyone learning about opal is the difference between precious opal and common opal.

Precious opal displays play-of-color.

Common opal does not.

That does not mean common opal cannot be attractive or valuable as jewelry material. Common opal occurs in numerous body colors and may display beautiful translucency, patterns, and soft coloration.

Pink, yellow, white, blue, green, orange, and other varieties can all be desirable even without the shifting spectral colors associated with precious opal.

Understanding this difference also prevents a common misconception: an opal does not have to display a rainbow of colors to be genuine opal.

What Determines an Opal's Appearance?

When examining precious opal, gemologists and experienced buyers consider considerably more than whether the stone contains color.

The body color provides the background against which play-of-color appears. Dark backgrounds can create dramatic contrast, which is one reason fine black opal is so highly regarded.

The brightness and range of play-of-color also matter. Some specimens display small pinpoints of color, while others reveal broad flashes or distinctive patterns across much of the surface.

Pattern, transparency, color distribution, stability, size, and the quality of cutting can all influence how an individual opal is appreciated and valued.

Two opals of identical weight can therefore look—and be valued—very differently.

Where Does Opal Come From?

Australia has historically been one of the world's most important sources of precious opal and is particularly famous for black, white, light, and boulder opal.

Ethiopia has also become a major source of gem-quality opal, including material known for vivid play-of-color. Other deposits occur in Mexico, Brazil, the United States, and several additional regions.

Origin can influence an opal's appearance and physical behaviour. For example, some opals are hydrophane, meaning their porous structure can absorb water. This characteristic is particularly associated with some Ethiopian material.

For that reason, knowing an opal's origin can sometimes provide useful information about how the stone should be handled and cared for.

Is Opal Durable?

Opal measures approximately 5.5 to 6.5 on the Mohs hardness scale. That makes it softer than gemstones such as quartz, sapphire, and diamond.

However, hardness measures resistance to scratching—not the complete durability of a gemstone.

Opal can also be sensitive to impacts, abrupt temperature changes, and extremely dry conditions. Rings and bracelets generally experience more contact during everyday wear than pendants and earrings, so the setting and intended use should be considered when selecting opal jewelry.

With thoughtful handling, opal can remain beautiful for many years.

How Should Natural Opal Be Cared For?

Gentle cleaning is usually the safest approach.

Use a soft damp cloth or mild soap and lukewarm water when appropriate. Avoid harsh chemicals, extreme temperature changes, and abrasive cleaners.

Ultrasonic and steam cleaners are generally best avoided because opal's structure and possible treatments can make aggressive cleaning methods unsuitable.

Extra care is also necessary with assembled opals such as doublets and triplets, which we'll examine later in this series.

Why Opal Is Unlike Almost Any Other Gemstone

Opal challenges many of the assumptions people have about gemstones. It does not possess a conventional crystalline structure. It contains water. It can be transparent, translucent, or opaque. It occurs in numerous body colors, and some specimens produce spectacular spectral flashes while others show no play-of-color at all.

That diversity is exactly what makes opal such an interesting subject for gemologists, collectors, jewelry designers, and anyone curious about natural gemstones.

And it leads to an important question: if opal can appear in so many different forms, how do you tell one type from another?

In Part 2 of our Opal Learning Series, we'll look closely at black opal, white opal, boulder opal, crystal opal, fire opal, and other important varieties—and explain what actually makes them different.