Opal is a unique and captivating gemstone distinguished by its vibrant display of spectral colors, a phenomenon known as opalescence. This beautiful effect arises from its composition as a hydrated amorphous form of silica, or silicon dioxide combined with water. Unlike many other popular gems, opal possesses a relatively delicate physical nature. Understanding the gem’s specific hardness rating and the structural reasons behind it is necessary for preserving its characteristic beauty over time.
Understanding the Mohs Scale of Hardness
Gemstone hardness is a measurement of a material’s resistance to scratching or abrasion. This property is quantified using the Mohs scale of hardness, a qualitative scale that ranges from 1 to 10, indicating relative scratch resistance. The scale is based on the ability of a harder material to visibly scratch a softer one. Talc sits at 1, representing the softest mineral, while diamond, the hardest known natural material, is positioned at 10. Minerals with a higher number can scratch all those with a lower number. Common quartz, which is a component of household dust, ranks at 7, providing a crucial benchmark for the general wearability of any given gemstone.
Opal’s Specific Hardness Rating and Structure
Opal typically ranks between 5.5 and 6.5 on the Mohs scale of hardness, placing it in the softer range of gemstones used in jewelry. This moderate rating means the gem is susceptible to scratching by materials that are harder than 6.5, including most common jewelry stones like sapphire (9) and the quartz particles present in everyday dust. The primary reason for this comparatively low hardness lies in opal’s non-crystalline, or amorphous, structure. Opal is a mineraloid, meaning its internal arrangement of silica spheres lacks the ordered, repeating atomic lattice found in true minerals. This disordered structure makes the material less resistant to physical abrasion. Furthermore, opal contains a significant amount of water, which can range from 3% to 21% by weight. This inherent moisture content further contributes to the gem’s delicate nature.
Daily Care and Considerations for Opal Jewelry
The Mohs hardness rating of 5.5 to 6.5 directly dictates the necessary care required for opal jewelry, especially rings, which are prone to impact. It is advisable to choose protective settings, such as bezels, which completely surround the gem’s edges, rather than prongs, which leave the sides exposed to potential knocks and scratches. Owners should remove opal jewelry before engaging in activities that involve rough contact, like gardening, cleaning, or sports.
Opal’s water content presents a unique durability concern beyond mere hardness, as rapid temperature fluctuations or prolonged exposure to very dry environments can cause dehydration. This loss of internal moisture can lead to a phenomenon called crazing, where a network of fine cracks develops on the gem’s surface or interior, permanently damaging its appearance. For this reason, it is important to avoid exposing opals to extreme heat, such as boiling water or direct sunlight for extended periods.
Due to the internal water and composite nature of certain varieties, such as doublets and triplets, cleaning methods must be gentle. Harsh chemicals and abrasive cleaners should never be used on opals, and ultrasonic cleaners must be avoided entirely, as their intense vibrations can cause solid opals to crack or cause water penetration in composite stones. Instead, cleaning should be limited to wiping the surface gently with a soft cloth and mild, lukewarm soapy water, followed by immediate, careful drying.