“Wow!” is what people want to hear after installing onyx. According to Richard McKenzie of Ocean+ Merchant, the stone is thought of as being a fairytale stone – it becomes what you want it to be. It can be used to add a sense of mystery, set the mood or make a statement. It is indeed a special stone in that it makes people stop and take notice.
Onyx or Alabaster?
The use of the term ‘onyx’ to describe the large variegated dimension stone slabs we see in stone shops is a misnomer. The stone type commonly referred to as onyx in the dimension stone industry would more appropriately be called alabaster. This stone type is formed by chemical precipitation of calcite in a similar manner to stalactites and stalagmites found in limestone caves; but this process does not produce the large slabs required for dimension stone production.
Alabaster is also produced in a similar manner to travertine when carbonate-saturated water resurfaces at springs and the change in pressure and temperature results in the release of the carbon dioxide causing precipitation and crystallisation of the calcite. Its deposition in successive layers (along with minor mineral contaminants) gives rise to the banded appearance that the stone often shows in cross section. Alabaster is relatively soft with a Mohs hardness of around 3 (can be scratched easily with a knife).
True onyx is a variety of chalcedony which is a siliceous mineral where the silica crystals are so fine they are difficult to distinguish under the microscope (referred to as “cryptocrystalline”). Onyx can exhibit banding due to variations in the crystal structure of the silicon dioxide. Being composed of silicon dioxide, ‘true onyx’ is much harder than alabaster with a Mohs hardness of around 7 (can’t be scratched easily with a knife).
So, the use of the term ‘onyx’ to describe a dimension stone slab is mineralogically incorrect, or at a least, a misleading term. To provide some clarification between the two distinct stone types the dimension stone product sold as onyx should be called alabaster or alabaster-onyx.
Alabaster-onyx is typically highly figured and its appearance differs from bands to swirls depending on slabbing orientation. The stone comes in a broad range of colours but typically has a milk-white to light yellow-brown background colour with varying bands or swirls of other colours of such as red, green or gold depending on the trace minerals present.
The fine-grain nature of alabaster-onyx allows the stone to be cut and ground to a high polish and makes the stone suitable for carving into intricate statuary. Alabaster-onyx has a relatively high flexural strength and can be cut into slabs thin enough to bring it to life by revealing its translucent properties. The translucent nature of onyx is the defining quality which sets it apart from other dimension stones and ensures its reputation as “the fairytale stone”.
Although the stone’s properties make it relatively versatile, its use in the dimension stone industry is usually limited to feature elements such as backlit walls and counter tops. It could also be used for floor tiles in low traffic areas.
Alabaster-onyx is predominantly composed of calcium carbonate which can be etched by acidic substances such as wine, soft drinks and some liquid soap. If the stone is likely to be exposed to acids the use of a honed or matt surface will make etching less conspicuous. It is important to note that the application of an impregnating sealer will not prevent etching as they do not protect the surface of the stone.
Much of the alabaster-onyx slabs on the market contain fractures of some kind and are usually reinforced by the application of netting on the back face. The presence of these fractures compromises the structural integrity of the stone and larger formats are often laminated onto glass, acrylic or polycarbonate sheet. The use of components with significantly differential rates of thermal expansion can lead to the development of shear stresses which may cause delamination. Therefore the laminated product needs careful design consideration especially where the stone may experience temperature fluctuations.
Alabaster-onyx has a low abrasion resistance which can lead to loss of polish if used as floor tiles in high traffic areas.
Specification and Testing
There is not a standard specification for alabaster-onyx. The stone is primarily used as a decorative element in protected locations and does not usually have a structural purpose. As alabaster-onyx is primarily composed of calcite, the standard specification ASTM C503 for marble dimension stone may represent a basic guide for selection.
Alabaster-onyx has a much finer crystal structure than marble which suggests that the criterion for water absorption should be lower than that set in ASTM C503. The standard provides some separate requirements for calcite marble.
The specification of any dimension stone should be based on location, design and engineering considerations specific to the intended use. The specification states a minimum strength requirement which requires the determination of both dried and soaked strength.
Water absorption and flexural strength are the key performance indicators for this stone and should be evaluated closely throughout the project supply phase to ensure adequate performance in service. Variations in water absorption and flexural strength are likely to indicate the presence of cracks in the stone.
Alabaster-onyx is available in a wide range of colours and figuring that allows you to make the unique statement you want. Taking note of the following points will assist you in the selection of the right stone for the job and its maintenance well into the future:
- Use water absorption and flexural strength tests as key performance indicators.
- Alabaster-onyx is acid sensitive so it is important to clean up spills immediately to avoid permanent etching.
- If a high risk of accidental spills exists, consider using a coating in preference to a sealer, especially in public spaces.
- The stone is relatively soft and is therefore prone to scratching and loss of polish in high traffic areas.
- Slabs may contain cracks so reinforcing or lamination may be required if the finished elements are exposed to flexural loads.
Article by Jim Mann, stone consultant and principal of Stone Initiatives.