There are many different types of gemstones that come in a wide variety of colors. However, one gemstone in particular is known for displaying multiple hues in a single stone. This gemstone is the opal. Opal is unique among gemstones due to its ability to showcase an array of spectral colors in one specimen. Let’s take a closer look at how opal exhibits this phenomenon.
The Structure of Opal
Opals are composed of silicon dioxide and water. The water content can range from 3% to 21% but is usually between 6% and 10%. The structure of opal is made up of silica spheres that are stacked together in an irregular pattern with the water filling in the gaps between the silica layers. This arrangement is known as a non-crystalline or amorphous solid.
The varying size of the silica spheres and their haphazard arrangement is what allows opal to diffract light and produce flashes of color. When light enters an opal, it bends and splits into the different spectral colors as it hits the layers of silica spheres. This process is referred to as diffraction.
Types of Opal
There are several varieties of opal based on their background color or appearance:
| Type | Description |
|---|---|
| White or Light Opal | Translucent to opaque with a white or light body color |
| Black Opal | Translucent to opaque with a dark gray to black body color |
| Fire Opal | Transparent to translucent with a yellow, orange, or red body color |
| Boulder Opal | Opaline material embedded in its host rock matrix |
The most prized and valuable variety is black opal. These display bright spectral flashes against the dark body tone. Fire opal is also highly desired for its fiery orange to red body color.
What Causes the Play of Color?
The play of color refers to the flashes of spectral hues seen in opal. As light travels through the silica spheres, the differing sizes cause the light rays to bend and disperse into the visible color spectrum. The silica layers create interference that brings out these colorful displays.
The play of color depends on the size, orientation, and uniformity of the silica spheres. More haphazard structures create increased diffraction and brighter flashes of color. The play of color varies based on the viewing angle. Tilting the opal back and forth will reveal different color patterns.
Common Opal Colors
Opal can display practically every color of the visible light spectrum. Here are some of the most common color flashes seen:
| Color | Description |
|---|---|
| Red | Scarlet, cherry, fire engine, ruby red |
| Orange | Salmon, peach, tangerine, amber |
| Yellow | Canary, lemon, golden |
| Green | Emerald, lime, forest, mint |
| Blue | Aqua, teal, cobalt, sapphire |
| Violet | Magenta, lilac, orchid, grape |
Reds, oranges, and yellows are the most common secondary hues seen in opal. Blues and violets are less common but particularly prized.
Some specimens can show a wide spectrum of colors in a single stone. Others may display only two or three colors repeatedly. Every opal is unique in its exact play of color.
What Influences the Color Display?
Several factors determine the type and intensity of colors shown in an opal:
– Thickness – Thicker opals allow more diffraction to occur, creating more color flashes. Thinner materials have fewer color variations.
– Uniformity – More organized silica spheres reduce diffraction and show less play of color. Irregular spheres increase the spectral display.
– Viewing angle – The play of color changes based on the angle at which light enters the opal.
– Background color – Darker body tones, like black opal, make the color flashes more vivid. Lighter body colors showcase pastel hues.
– Cut style – The pattern and shape the opal is cut into impacts the color display. Curved cuts maximize play of color.
– Lighting conditions – Direct sunlight versus shaded light brings out different color mixes.
Famous Opals
Some of the most famous opals display an unrivaled play of color and beauty. Here are three legendary specimens and their unique traits:
| Opal Name | Description |
|---|---|
| The Aurora Australis | One of the most valuable opals in the world, displaying red, green, blue, and violet flashes against a black body. |
| The Andamooka Opal | Considered the finest crystal opal ever discovered with huge broad flashes of color. |
| The Olympic Australis | The largest and most valuable opal ever found, weighing 17,000 carats uncut. |
These world-class opals demonstrate the full spectrum of colors possible in fine specimen gemstones. Their rarity and unparalleled beauty make them invaluable.
The Rarity of Opal
High quality opal is very rare. Opal rates at around a 6.5 on the Mohs hardness scale, making it a relatively soft and delicate gemstone. Fine opals are difficult to extract intact from the host rock. The delicate structure also makes opal prone to cracking or crazing over time.
Opals form under very specific conditions. The silica spheres can only settle into orderly layers with the right combination of water saturation, silica concentration, temperature, and pH. These stringent requirements limit opal formation and preserve the rarity of precious stones.
Australia produces over 90% of the world’s opal. Other mining locations include Brazil, Mexico, the United States, and Ethiopia. The rarity and concentration of supply maintain the value of superior quality opal.
How to Judge Opal Quality
Several factors determine the value and price of individual opal specimens:
– **Play of Color** – The number of colors, dominance of reds/greens/blues, and vibrancy or intensity denote quality.
– **Pattern** – Flames, strips, flecks, spiral, and harlequin patterns are most desirable. Pinfire and Chinese writing are less valued.
– **Background Color** – Black opal commands the highest prices followed by white or light. Fire opal is also valuable.
– **Clarity** – Transparent to translucent clear material with few inclusions or flaws.
– **Cut** – Smooth domed backs and expert faceting to maximize play of color.
– **Carat** – Larger uncut, flawless stones worth more than tiny specimens.
– **Body Tone** – Vivid play of color against black or colored base is ideal. Chalky opal is less prized.
Opal is a stunning yet delicate gemstone prized for its signature play of color. No two opals are exactly alike in their spectral display. It remains one of the most mesmerizing and beautiful gemstones on Earth.
Conclusion
Opal stands apart as the only common gemstone that exhibits a dynamic play of color in a single stone. This optical phenomenon results from the unique structure of silica spheres suspended in water that diffract light. Opal can display practically every hue in the visible spectrum, with fiery reds and vibrant greens being most coveted.
The quality of the play of color along with factors like clarity, carat, and cut determine the value of individual opal specimens. No two opals are exactly the same in their color display, making each one a unique work of natural art. Opal’s rarity, fragility, and dazzling flashes ensure it remains one of the world’s most spellbinding gemstones.
