Opals captivate with their unique display of shifting colors, a phenomenon known as play-of-color. These gemstones derive their visual appeal from microscopic silica spheres that diffract light. Understanding their physical properties, particularly hardness, is important for appreciating their beauty and ensuring longevity. This article explores opal’s position on the mineral hardness scale and what that means for owners.
Understanding Mineral Hardness
Mineral hardness refers to a material’s resistance to scratching or abrasion. The Mohs scale of mineral hardness, developed by Friedrich Mohs in 1812, provides a qualitative measure of this property. This scale ranks minerals from 1 to 10 based on their ability to scratch softer materials. For example, talc is rated 1, while diamond, the hardest known natural mineral, is rated 10.
A material with a higher Mohs number can scratch any material with a lower Mohs number. A fingernail typically has a hardness of about 2.5, and a steel knife blade is around 5.5. Quartz, a common mineral found in dust, ranks at 7 on the Mohs scale.
Opal’s Position on the Hardness Scale
Opals typically exhibit a hardness ranging from 5.5 to 6.5 on the Mohs scale. This places them in a category of gemstones softer than many commonly worn jewelry stones. Their unique amorphous structure, rather than crystalline, contributes to this relative softness. Unlike minerals with a rigid, repeating atomic lattice, opals consist of hydrated silica spheres.
This amorphous structure, combined with their significant water content—ranging from 3% to 21% by weight—makes opals susceptible to scratching and impact. For comparison, gemstones like sapphire and ruby register at 9 on the Mohs scale, while diamonds are a 10. Opals require more careful handling than these harder alternatives and are prone to damage from sudden temperature changes or dehydration.
Caring for Opals
Opals require specific care to maintain their integrity and appearance. To prevent scratches, avoid wearing opal jewelry during activities that might expose it to impacts or rough surfaces, such as gardening or sports. Store opals separately from other jewelry, especially harder gemstones, to prevent scratching.
Opals are also susceptible to chipping or cracking if dropped or subjected to sharp blows. Their water content means they can be affected by extreme temperature changes or very dry environments. Prolonged exposure to intense heat or direct sunlight can cause dehydration, potentially leading to “crazing,” where fine cracks appear on the surface. Therefore, keep opals away from heat sources and maintain some humidity.
Clean opals gently using a soft cloth and mild soap with lukewarm water. Harsh chemicals, abrasive cleaners, or ultrasonic cleaners can damage the stone’s delicate structure and affect its water content. Avoid soaking opals for extended periods, and thoroughly dry them with a soft cloth after cleaning.