Opals are known for their mesmerizing “play-of-color,” but they are sensitive to heat and prolonged light exposure. The primary concern with wearing opals in the sun is not the light itself, but the accompanying heat energy. Opals contain a significant amount of water within their structure, and exposure to high temperatures from direct sunlight can cause the stone to dry out, leading to permanent damage. Therefore, while momentary exposure is generally harmless, prolonged time in bright, hot sun should be avoided to protect the gem’s integrity.
The Role of Water in Opal’s Physical Structure
The visual characteristics of opal stem from its composition as a hydrated form of amorphous silica, meaning it lacks a defined crystalline structure. Chemically, opal is silicon dioxide with a water content that typically ranges between 3% and 10% by weight, though some varieties may hold up to 21% water. This water is incorporated directly into the stone’s physical makeup, classifying it as a mineraloid rather than a true mineral.
The signature flashes of color, known as play-of-color, are created by the internal arrangement of microscopic silica spheres. These uniform spheres are stacked in an orderly, three-dimensional array, which acts like a natural diffraction grating. As white light enters the opal, it is split into its component colors, which then flash and shift as the stone is moved.
The stability of the opal is fundamentally dependent on maintaining its natural water content. If the water begins to evaporate, the internal structure of the stone becomes unstable, putting the silica sphere arrangement at risk. The water acts as an internal stabilizer, ensuring the structural framework remains consistent and capable of diffracting light properly.
Direct Impact of Sunlight: Dehydration and Crazing
Sunlight poses a threat to opals primarily because of the heat it delivers, which accelerates the process of dehydration within the stone. When an opal is exposed to direct, intense solar radiation, the internal temperature of the stone increases. This thermal energy provides the necessary force for the water molecules trapped within the silica structure to evaporate quickly.
As the water leaves the stone, the overall volume of the opal shrinks, but this shrinkage is often uneven, creating immense internal stress. This stress eventually manifests as a network of fine, hairline cracks on the surface and within the body of the gem, a condition known as crazing. Crazing is a permanent and irreversible form of damage that significantly reduces the stone’s brilliance.
Water loss also directly affects the opal’s visual display, leading to a dulling or complete loss of the play-of-color. The orderly spacing between the microscopic silica spheres is disrupted by the structural shrinkage, which changes how light is diffracted. This loss of internal structure diminishes the stone’s characteristic optical effect, leaving the opal looking milky or lifeless.
While all opals are susceptible, those with higher initial water content, such as certain hydrophane opals, can absorb and expel water more rapidly. This makes them sensitive to sudden environmental changes and requires extra care.
Comprehensive Care for Opal Jewelry
Protecting opal jewelry focuses on minimizing exposure to heat and avoiding rapid changes in temperature and humidity. Avoid leaving opals in high-heat environments like near a window, under direct display lights, or inside a vehicle on a warm day. Occasional wear outdoors is acceptable, but prolonged sunbathing or working in intense heat should be avoided to prevent accelerated dehydration.
For storage, opals should be kept in a stable environment rather than a hot, dry space like a safe or bank vault. If you live in an especially dry climate, you can provide moisture by storing the opal in a sealed plastic bag with a slightly damp cotton ball or cloth. This helps maintain the necessary internal hydration and prevents the stone from becoming brittle.
When cleaning, only use mild, lukewarm water and a soft cloth to gently wipe the stone. Harsh chemicals, bleach, or ammonia can damage the opal’s delicate surface. Never use ultrasonic cleaners or steam cleaners, as the intense vibrations and heat can cause the internal water to boil or expand, leading to crazing.