The strength of onyx depends entirely on the type of durability being measured. The onyx used in architecture and design, often called “onyx marble” or banded calcite, is composed primarily of the mineral calcite, which is a form of calcium carbonate. This composition gives the stone its unique translucency and striking patterns, but it also dictates its overall resilience to physical and chemical wear. Understanding the true strength of onyx requires looking at its resistance to scratching, its structural integrity, and its vulnerability to common household chemicals.
Scratch Resistance and the Mohs Scale
The resistance of onyx to surface damage is relatively low due to its calcite composition. Calcite measures approximately 3 on the Mohs scale of mineral hardness, although commercial onyx can sometimes be rated slightly higher, around 4 to 5. This low ranking means that many common objects found in a home or commercial setting can easily cause visible scratches on the surface of onyx.
For context, a simple copper penny is slightly harder than calcite, and common window glass or a pocketknife blade is rated at around 5.5. A scratch from a dropped ceramic dish or a piece of loose grit from a shoe is highly likely to damage an onyx surface. In contrast, common countertop materials like granite typically rate between 6 and 7, and engineered quartz is similarly hard, making them much more resistant to everyday abrasion. This inherent softness means onyx requires careful handling and is best suited for low-traffic or decorative applications where its surface is protected.
Structural Load Bearing Capacity
The ability of onyx to withstand crushing or breaking forces is measured by its compressive and flexural strength. Compressive strength, the force required to crush the stone, is relatively high for onyx, typically ranging from about 940 to 1,760 kilograms per square centimeter. This range indicates that onyx is strong enough for most vertical cladding or flooring applications where the load is distributed evenly and directly downwards.
The stone’s flexural strength, which is its ability to resist breaking when bent, is significantly lower and is a practical concern for installations like unsupported countertops. Flexural strength values for onyx are much lower, sometimes reported between 54 and 125 kilograms per square centimeter. This lower tensile resistance means that onyx slabs used horizontally, especially in thin sections, are prone to cracking or snapping if they are not adequately supported beneath the surface. Furthermore, because onyx is primarily calcite, the stone has a property called perfect cleavage, which means it has planes of weakness where it is inherently more likely to fracture under stress.
Resistance to Staining and Etching
The durability of onyx against chemical and environmental factors is a concern, primarily due to its chemical makeup and physical structure. Onyx is a porous material, which means it has microscopic voids that can absorb liquids. This porosity makes the material highly susceptible to staining from common liquids like cooking oils, wine, or coffee, necessitating the application of a penetrating sealer to minimize absorption.
A more immediate weakness is its vulnerability to etching. Since onyx is calcium carbonate, it reacts strongly with any acidic substance it encounters. Common household items such as vinegar, lemon juice, soda, and many bathroom cleaners will instantly cause a chemical reaction that dissolves the calcite minerals on the surface. This chemical attack leaves behind a permanent dull, rough mark known as an etch, which is a permanent alteration of the stone’s finish. Etching cannot be removed with cleaners and requires professional re-polishing to restore the surface integrity.