Onyx is a beautiful, banded stone frequently chosen for its rich colors and unique translucent qualities in both jewelry and decorative architecture. This material is a form of chalcedony, a microcrystalline variety of the mineral quartz, with its characteristic parallel banding caused by silica deposition during formation. While its elegance is undeniable, understanding its physical properties is necessary to determine its suitability for various uses. Compared to many other decorative stones like granite or quartzite, onyx possesses an average to moderately soft hardness level.
Understanding Hardness: The Mohs Scale
Mineral hardness is quantified using the Mohs scale. This scale ranks minerals from one (talc) to ten (diamond) based on their ability to scratch one another. It is a relative scale; a material with a higher number can scratch any material with a lower number. The scale does not measure a stone’s resistance to breaking or chipping, which is known as toughness.
True onyx, being a quartz-based material, typically rates between 6.5 and 7 on the Mohs scale of hardness. This places it in a respectable range, indicating inherent scratch resistance against common household dust, which is largely composed of silica particles. The hardness level is directly attributable to the stone’s primary composition of silicon dioxide.
Practical Durability and Scratch Resistance
Translating the 6.5 to 7 Mohs rating reveals that onyx is generally durable enough for many applications, but it is susceptible to damage from harder materials. Objects like steel knives, which are usually rated around 5.5, or glass, which is around 5.5 to 6, may struggle to scratch true onyx. However, the stone is softer than common countertop materials like granite or engineered quartz, which often rate at 7 or higher. This distinction is important when considering its use for high-traffic surfaces.
Onyx is widely used for jewelry, such as pendants and earrings, where it rarely encounters abrasive conditions and is quite suitable for daily wear. Rings, however, may be more prone to scratching from harder stones or accidental impacts, requiring more mindful use. For architectural applications, like countertops or flooring, the stone’s moderate hardness means it can be scratched by abrasive dirt or grit tracked in from outside. Using cutting boards and avoiding dragging objects across the surface is necessary to preserve its polished finish.
A significant source of confusion regarding durability stems from the widespread mislabeling of a softer stone as onyx. Materials commercially sold as “Mexican Onyx” or “Onyx Marble” are actually banded calcite, a calcium carbonate mineral. This calcite material only rates a 3 to 4 on the Mohs scale, meaning it can be easily scratched by a simple steel knife or even a copper coin.
Caring for Onyx: Maintenance and Protection
Onyx requires specific care to maintain its appearance due to its moderate hardness and inherent porosity. The stone’s structure makes it vulnerable to staining from liquids, especially when used in slab form for surfaces like vanities or bar tops. For these architectural uses, applying a high-quality sealant regularly is necessary to fill the microscopic pores and prevent the penetration of spills.
Cleaning methods must be gentle, focusing on mild, pH-neutral detergents and soft cloths to avoid abrasion. The use of acidic cleaners, such as vinegar, lemon juice, or many common household sprays, should be strictly avoided as they will cause etching on the stone’s surface. This chemical damage results in dull spots that are particularly noticeable on a polished finish.
For onyx jewelry, proper storage is a preventative measure against scratches. It should be kept separate from harder gemstones, especially diamonds, which could easily abrade the surface. Furthermore, due to its porous nature, onyx jewelry should not be soaked in water or harsh cleaning solutions, as prolonged exposure can affect its appearance and integrity.