Opal is a captivating gemstone, chemically defined as a hydrated amorphous form of silica. This structure, which contains water molecules, results in the stone’s characteristic “play-of-color.” However, it also makes opal significantly more fragile than many other stones used in jewelry. Opal is considered a relatively soft and delicate gemstone compared to hard stones like diamond or sapphire.
Measuring Durability: Opal on the Mohs Scale
The standard method for assessing a gemstone’s resistance to scratching is the Mohs Hardness Scale, which ranks minerals from 1 (softest) to 10 (hardest) based on which material can visibly scratch another. Opal typically scores between 5.5 and 6.5 on this scale, placing it in the moderate range of hardness. For context, the hardest materials often used in fine jewelry, such as diamond, score a 10, and sapphire scores a 9. Even common quartz, the primary component of ordinary dust and sand, registers a 7 on the Mohs scale, meaning that simply wiping dust off an opal without a soft cloth can cause microscopic scratches over time.
A hardness rating below 7 is generally considered a vulnerability for stones set in rings, which are constantly exposed to bumps and abrasion. This difference in scratch resistance highlights why an opal ring requires more careful wear than a ring set with a harder stone. While opal is harder than some organic gems like pearl (around 2.5), its position on the scale confirms its susceptibility to surface damage from everyday contact with harder objects.
Structural Instability: The Role of Water Content and Crazing
Opal’s fragility extends beyond its surface hardness due to its unique internal chemistry. The stone is composed of hydrated silica, and its water content can range from 3% to 21% by weight, with gem-quality opals usually containing between 6% and 10% water. This molecular water is trapped within the microscopic structure of silica spheres, and its presence is the source of the opal’s distinctive optical properties.
A major concern for opal owners is a type of structural failure known as “crazing” or “checking,” which manifests as a network of fine cracks on the stone’s surface or within its body. Crazing occurs when the opal loses its internal water too rapidly, causing the stone to shrink unevenly and create internal stress. Exposure to high heat, low humidity, or sudden, significant temperature fluctuations—known as thermal shock—can accelerate this dehydration process. This cracking is a structural problem distinct from simple scratching and can permanently damage the stone’s appearance.
Essential Care Practices for Opal Jewelry
Given opal’s moderate hardness and sensitivity to dehydration, adopting specific care practices is important for maintaining its condition. When wearing opal jewelry, especially rings or bracelets, choose protective settings like bezels, which fully encase the stone’s delicate edges. Remove opals before engaging in activities that involve physical impact, such as sports, gardening, or household chores, to prevent chipping or scratching.
For cleaning, use only warm water and a mild soap solution applied with a soft, lint-free cloth. Steam cleaners or ultrasonic cleaners must be avoided, as the intense vibrations and heat can easily cause crazing or cracking by inducing thermal shock. Harsh chemical cleaners, such as bleach or ammonia, should also be avoided because they can attack the stone’s surface.
Opals should always be stored separately from other jewelry to prevent harder stones like diamonds or sapphires from scratching their surface. Storing the stone away from direct heat sources and in an environment with stable, moderate humidity helps prevent the rapid water loss that leads to crazing.