Opals are stunning gemstones that exhibit a colorful iridescent play-of-color. This rainbow-like display is caused by the internal structure of the opal, which contains microscopic silica spheres in an orderly grid-like pattern. When light enters an opal, it diffracts off these silica spheres, separating the light into its spectral colors. This makes opals glow with mesmerizing flashes of color that shift and change as the gemstone is viewed from different angles.
What causes opals to become clear?
In rare cases, opals can become clear and lose their prized play-of-color over time. There are a few potential reasons this may occur:
- Loss of water content – Opals contain between 3% to 21% water trapped within their silica structure. Over time, especially if not stored properly, an opal may dry out and lose some of this water. This causes the silica spheres to shrink, collapse, and lose their orderly arrangement. Without the intricate silica grid, light no longer diffracts into spectral colors.
- Heat damage – Exposing an opal to high temperatures can also disrupt its silica structure. Heating causes the water to evaporate rapidly. Additionally, the silica spheres may expand at different rates, disrupting their grid-like arrangement. Prolonged heat exposure, such as through submersion in hot water, can render an opal clear and opaque.
- Irradiation – Treating opals with radiation, such as gamma rays, x-rays or ultraviolet light has been found to remove their play-of-color. The high-energy radiation knocks water molecules out from the silica structure and damages the orderly arrangement of the silica spheres.
- Mechanical damage – Any chipping, fracturing or abrasion that affects the internal structure of the opal can also cause it to become clear. This type of damage randomly scatters the silica spheres, preventing the diffraction of light necessary to produce opalescence.
Can a clear opal regain its play-of-color?
In some cases, it may be possible to restore an opal’s play-of-color after it has become clear:
- Rehydration – If an opal lost its play-of-color due to dehydration, soaking it for weeks or months in water may help it reabsorb water and regain its silica structure and opalescence. This technique works best on opals that have started showing signs of crazing (fine surface cracks).
- Surface polishing – If the loss of opalescence is due to abrasions or surface damage, carefully polishing away damaged surface layers may reveal undamaged opalescent material underneath. However, polishing also removes some of the stone, so stones must be thick enough to withstand the material removal.
- Irradiation treatment – Low-dose x-ray or gamma ray irradiation has been used to enhance or restore the play-of-color in some types of opals. However, this is risky, as improper or prolonged irradiation can further damage the opal. Color changes may also be unpredictable.
Unfortunately, if an opal’s structure is severely damaged by extensive dehydration, fracturing, or prolonged heat exposure, the play-of-color is likely lost forever. The only option may be to cut and polish the clear stone into a more traditional cabochon or faceted shape.
Common causes of opal damage
To prevent damage that may ruin an opal’s precious play-of-color, gemologists recommend avoiding the following:
- Cleaning with ultrasonic or steam cleaners – The intense vibrations and/or heat can fracture the opal’s silica structure.
- Submerging in water for prolonged time – While brief water immersion generally does not harm opals, soaking for weeks or months may lead to crazing and dehydration damage.
- Exposing to sudden temperature changes – Rapid heating or cooling can cause fracturing due to expansion or contraction of the silica spheres.
- Wearing in rings or bracelets – Constant impact and abrasion against hard surfaces will eventually cause fracturing and surface damage.
- Storing in direct sunlight – Prolonged exposure to UV radiation can degrade the opal’s silica structure and cause discoloration.
- Exposing to chemicals – Household chemicals, especially acidic compounds like lemon juice or vinegar, can etch and erode the opal’s surface.
With proper care and storage, most opals will retain their magical play-of-color for many years. Avoiding water immersion, heat, chemicals, and mechanical impact will keep their intricate silica structure intact and their colors bright.
Types of opals and their care
Not all opals have the same stability and care requirements. Here is a breakdown of the most common opal varieties:
| Opal Variety | Water Content | Stability | Care Recommendations |
|---|---|---|---|
| Precious opal | 3-10% | Low | Avoid water immersion, heat, harsh chemicals |
| Fire opal | 3-8% | Moderate | Take care to avoid knocks and fractures |
| Common opal | 3-15% | Good | Protect from abrasions and acids |
| Peruvian opal | 5-20% | Moderate | Avoid overheating and rapid temperature changes |
| Matrix opal | 5-21% | Good | Protect from mechanical damage |
As shown, precious opal has the lowest water content and stability. Matrix opals, which have a protective surrounding rock matrix, tend to be one of the most durable and resilient to damage. Understanding the characteristics of each opal type will allow proper care.
Other causes of opal color change
While most opal color changes are due to structural damage, there are some other natural causes that may alter an opal’s appearance over time:
- Oxidation – Exposure to oxygen can sometimes lead to faint yellowish or brownish tints. This does not damage play-of-color but changes overall body color.
- UV light exposure – Prolonged, intense UV light can cause lightening or bleaching of darker opal body colors.
- Humidity – Hygroscopic opals absorb water vapor. Increased humidity intensifies play-of-color, while dry conditions mute it.
- Ambient lighting – Brightness and color temperature of ambient light impacts opal’s perceived play-of-color.
With care and minimal exposure to damage risks, most opals will retain their gorgeous iridescence for many decades. But improper care can quickly lead to a noticeable deterioration of their delicate play-of-color. Understanding what causes opal color change empowers collectors to protect these stunning gemstones for years of enjoyment.
Conclusion
The kaleidoscopic play-of-color in opals is a phenomenon of optics caused by the gemstone’s unique internal structure. However, this delicate arrangement of silica spheres is prone to disruption by dehydration, heat damage, irradiation, and mechanical fractures. Once damaged, the intricate structure that diffracts light into spectral flashes is lost, and the opal becomes clear. In some cases, proper care and rehydration treatments may help restore an opal’s play-of-color. But severe structural damage is likely permanent. By understanding what causes opals to lose their iridescence and handling them gently, collectors can maintain their captivating beauty for many years.
