‘Black granite’ remains one of the most popular choices for kitchen worktops. That is surprising, as there is no such thing as black granite. So what do we mean when we talk about black granite? Barry Hunt provides the answer.

In the construction industry, just say ‘black granite’ and everyone understands that you are talking about an exceptionally hard, particularly dark-coloured stone that is granite.

Well, I am sorry for this news, but there is no such thing as black-coloured granite. It just does not exist. Black granite is like a chimera; the sum of many other parts – parts that are other types of stone but certainly do not include granite.

A third to a half of the composition of granite is quartz, typically a translucent crystal that appears light grey. The other dominant mineral in granite is feldspar, which can be a range of colours but most commonly is white or grey, although shades of red, green and brown are not uncommon.

Quartz and feldspar are rarely black. It is only the trace minerals in granite, such as mica and amphibole, that have the really dark colours. So a black coloured granite is basically an impossibility in a geological sense.

The term ‘black granite’ is used because it is simple. The so-called ‘man on the street’ generally understands the notion of granite as a hard rock and, honestly, why should he be concerned with the many different kinds of rocks with igneous origin that appear similar to granite but actually are not? As far as he and the trade supplying ‘black granite’ is concerned any igneous rock can be lumped together under the general term of ‘granite’ and thus it follows that those of a black colour have become known as black granite.

What is ‘Black Granite’?

This might seem like a point to switch off and just accept that geologists have many different names for rocks that you are rarely if ever going to need to know.

But classification is not only important to geologists. It is now a requirement enshrined in law for all building stones to be properly classified, or identified by their true geological or petrographic name. Below is a list of some of the best known ‘black granites’ followed by the type of rock they actually are:

• Assoluto – Gabbro
• Pearl or Larvik Stone – Monzonite
• Finnish Black – Diabase
• Star Galaxy – Diorite
• Zimbabwe – Diorite
• Impala – Gabbro
• Angola Black – Dolerite
• Chinese G684 – Basalt

The common denominator here is that all these stones essentially have a low or non-existent quartz content and a high proportion of dark minerals loaded with iron and other metals. Therefore these stones also tend to be much denser than other igneous rock types, typically by 10-15%.

The lack of quartz and dominance of feldspar also make these stones easier to cut and polish, because feldspar is below quartz on the Mohs (or any other) scale of hardness.

These properties were identified by some early civilisations. The Egyptians used them to good effect for more realistic statues of African figures, for which some of the local basalts were employed (see the photo on the right of Amenemhat III).

Black granite is also found in the King’s Chamber of the Great Pyramid, in blocks weighing several tonnes each. For a time there was some controversy about how such blocks could have been cut and polished because it was believed there would have been no tools available to the masons of the day to achieve that. Current belief is that quartz sand was used as an abrasive aid to copper saws and drills – a method that has been demonstrated to be successful in recent times (albeit that the demonstration possibly used the wrong stone as it was considerably harder than the material in the tomb).

So we have a lot of different rock types collectively known as black granite to both the industry and the general populace. The term conveys a message, so why upset the apple cart?

Well, I have no intention of upsetting the apple cart, but you should remember that because what is called black granite could actually be any one of a number of materials, how it performs in use can be open to question. Different materials behave differently because they can be chemically and physically completely different.

For the ease of further discussion I will continue to call all these stones black granites unless I am singling out a specific stone type.

Black granites are found all over the world, mainly because basalt is the most ubiquitous of all rock types and just happens to be typically deep grey to black in colour.

It is common, but this does not mean it is easy to use in a commercial sense. Just head to Iceland, which is essentially a massive lump of basalt, and you will realise that the multiple layering and columnar structures create many issues that are difficult to overcome for potential dimension stone exploitation. Block sizes would be limited, while the typical hexagonal jointing would result in significant wastage in this particular instance.

It is unusual to find resources of black granite that can deliver blocks of sufficient size and with the necessary properties to be used as dimension stone.

Thus there is an apparently finite supply of good quality material. This is the root cause of many problems, because inferior materials are placed on the market in order to meet demand. Many reputable stone suppliers struggle to obtain what they know is suitable quality black granite, and lighter colour materials darkened with colour enhancers have been used (more about this below).

In recent years, the world supply of black granite has been dominated by China and India.

In China, the Shaanxii and Shandong provinces are where you find now well known stones such as Mongolian Black and, or course, Shaanxii Black.

In India there are extensive deposits of black granites, with the best quality coming from Kummam, Karsanur, Tindivanam, Paitur, Atur in Tamil Nadu, Charmarjanagar and Kollelgal in Karnataka and Warrangal in Andhra Pradesh. Lower quality materials are also found in these areas and there are lighter shades that may be regarded as being significantly inferior.

Price is very much based on the deepness of the black colouration. A finer grain size also generally increases the price, as does the absence of veining, spotting and other natural ‘irregularities’.

The UK is bereft of commercially viable black granite materials, although it does have a variety of resources. The Whin Sill that cuts through England and forms the base of much of Hadrian’s Wall would be a wonderful material to use but, unfortunately, it is crushed for aggregate in the one quarry where it is currently extracted.

The rock, known locally as whinstone, is a type of dolerite. It was formed by the intrusion of a hot liquid magma pushing between existing layers of rock. The magma cooled and crystallised to form the whinstone. The size of the resource is quite limited, which is typical of so many occurrences of such rock types. The rock also exhibits sharp changes in composition and properties closer to the edges, where it cooled more rapidly.

There are a few buildings local to the Whin Sill (in the hamlet of Holwick, for example) that have been built using the whinstone. They should outlast the surrounding limestone and sandstone buildings by centuries, but few were built as the stone was so hard and difficult to work.

Properties and Use

There are few uses of stone to which black granites could not be put. All the different kinds of black granite rocks typically have excellent physical, chemical and mechanical properties. Basically, think of good granite and then add a little to the strength and density and lower the porosity.

Table 1 (on the next page) includes some typical values for these types of stone, although it must be considered extremely general given the wide range of rock types that are represented in this category.

One of the toughest jobs for any stone is to be used as a road wearing course, and one location where black granite has proven itself in this application in the blockwork pavement of Exhibition Road in London.

This shared use scheme is one of the largest in Europe and undergoes continual heavy loading from buses and other heavy-axled vehicles through the course of every single day.

The stone was put through rigorous testing and appraisal ahead of construction and underwent continual on-site quality control while it was being laid to ensure no rogue quality materials were incorporated.

This is the essence of all stone construction: make sure the natural variation that all stone displays does not promote an adverse influence.

A more traditional use of black granite is for memorials, both because black is commonly associated with death and because of the contrast that can be created for lettering and imagery.

In more recent times this has been expanded with some of the larger national and international memorials that have been erected. The Vietnam Veterans Memorial in Washington DC is now ranked as one of America’s favourite pieces of architecture but was not without its controversy as it was being built.

It incorporates two gabbro walls, each 75m long and 3m high, which had 58,191 names inscribed on them when the monument was completed in 1983. More names have been added since then. The gabbro used came from Bangalore. It was selected because it was considered to exhibit the right reflective qualities for such a memorial.

Most people probably think of black granite in the kitchen, where worktops and splashbacks have become must-have items. It is difficult to go wrong with black, while the properties of the stone provide a scratch resistant, non-absorbent surface.

But, of course, it is in the kitchen where most problems seem to occur due to the high expectations of the users, the close scrutiny that such surfaces regularly undergo and the wide variety of chemicals (in cleaning products and food) that may be used on or around the stone.

By necessity, kitchen worktops should be of the highest quality. It is a false economy to select lower quality materials as these will rarely be truly loved the way such stone should be.

Fitness for purpose

Part of the problem of using such dark stone is the very thing that makes it so appealing to many: its consistency in appearance. Even the most minor irregularities, not noticeable with other stone types (or colours), can develop into major issues with customers. Polished finishes raise most issues as a consequence of the way light reflects off the surface, highlighting any differences.

The perception of colour is based on what wavelengths our eyes receive when objects are illuminated, usually by the sun but, of course, also the many forms of artificial light.

When white light hits the crystalline structure of the stone surface, some will reflect, some will be absorbed and some will just bounce around a bit (internally reflect) and eventually come back out, usually having lost some of the wavelengths.

That is how we perceive colour and depth of colour with stone. With a black stone, most of the light is absorbed, while for a white stone most makes it back out again.

If the surface is scratched, the light does not get absorbed by the stone but bounces off the faces of the broken crystals at the surface and is scattered as white light. If the stone is white the effect is negligible, but a black stone can appear grey if the surface is lightly roughened and almost white with heavy surface roughening.

This is why black granites are terrible for floors. They will suffer what is known as ‘matting up’, with surfaces becoming lighter and often exhibiting tracking where footfall is greatest – lift lobby areas in office buildings are particularly prone to tracking, which quickly becomes a major maintenance issue.

Buying polished black granite is a bit like buying a shiny black car: you notice every speck of dust and it can be hard ever to be satisfied that it is clean. You would never notice many of these ultimately trivial issues with most other stone types.

A common problem with work surfaces is light-coloured ring marks left when cups have been placed on the polished surface. They are difficult to clean away. One theory is that they are caused by a reaction between absorbed salts from dishwashing agents that enter the unglazed and porous ceramic on the bottom of cups and mugs. Some believe that even just the salts in water can cause the effect, although it is not usually seen when other wet items are placed on these surfaces.

Another phenomenon that received a small amount of press coverage a few years ago was the presence of what became known as ‘ink blots’. These are patches of significantly darker colour that some believed to be caused by the deposition of oily substances – something that could not occur naturally in such a stone type.

Many of these ‘ink blots’ were actually xenoliths, which are alien rock and mineral inclusions to the parent rock. They are absorbed by the rock when it is molten but have different concentrations of dark minerals and crystal grain size.

Some of the African black granites have suffered from these inclusions and the prices of different qualities may reflect the level of occurrence of these features, which are not always detectable until a worktop is installed and viewed under artificial light.

A simple trick to identify such inclusions is to take flash photographs of the slabs before they are processed. This usually highlights such features because of the different way in which the xenoliths absorb and reflect the bright light of the flash.

There is also a discolouration issue with the more basaltic stones that contain a significant amount of calcium carbonate or related minerals. The polish on such stones can be quickly removed by acids – and there are a lot of acids in foods.

This degradation of the polish has been observed on internal floors where there have been acid spillages such as wine and fizzy drinks and on external monuments and street furniture. Dare I say it, but bodily functions have also been responsible for the loss of polish at some public amenities.

Demand for black granite outstrips supply of the best material and good quality black granite has become harder to obtain. As a result there has been a gradual shift towards using lesser quality not-so-black granites and treating them with colour intensifiers or dark resins. But colour intensifiers wear off in use and maintenance, so the stone loses its deep, dark colour. And resins can eventually alter so that, again, there is a general lightening in the stone’s appearance.

The presence of resins can also prevent remedial actions from being taken by stopping the penetration of protective treatments.

Conclusion

Black granite is an enigma. It is difficult to know which face is being shown by the chimera within. It should all be so simple but, as with so many things that appear to be straightforward, especially when dealing with natural stone, the true state of matters is so much more difficult.

The best advice is to make sure you know where the stone comes from and that there have been no added treatments. And if you are going to make a black granite surface a star attraction, take out your camera and use the flash to see if it really has that star quality under harsh lighting.

The author – Barry Hunt

Barry Hunt is a chartered geologist, a surveyor and scientist. He has been awarded the designation of European Geologist and is a Corporate Building, Conservation and Specialist Surveyor. He is also a Member of the Chartered Institute of Building and a Fellow of the Royal Microscopical Society.

Barry has served on a number of professional committees, including the Technical Committee of Stone Federation Great Britain (SFGB), which provides advice on all stone construction issues. He is one of the authors of the SFGB team that has published codes of practice for the installation of stone floors and internal stone finishes.

Barry gained 14 years’ experience working as a consultant for two renowned civil engineering materials consultancies before, in 2001, establishing his own consultancy, IBIS, specialising in the investigation of construction materials.

The specialist knowledge and services provided by Barry have allowed him to be instrumental in the resolution of problems ranging from blast damaged claddings in London’s West End to advice on the quarrying and extraction of stone from abroad for import to the UK. Other areas of experience include the investigation of all types of building finishes, specialist advice on remedial treatments and the preparation of advice for potential and actual use in litigation or arbitration.

Having worked for consultancies that both undertook in-house laboratory investigation to UKAS requirements, Barry is also able to conduct or oversee a wide range of on-site and laboratory techniques and ensure they are carried out to traceable standards.

One speciality in all investigations is Barry’s hands-on approach. Being trained in industrial roped access (abseiling) allows him to get close to the problems with external building envelopes quickly, efficiently and cost-effectively.

Throughout his working life Barry has published findings from the many investigations he has undertaken. He has also authored chapters for two books on building stone and is currently engaged in other book projects in this field. His most important contribution is considered to be the chapter on the repair and maintenance of stone in the landmark Geological Society publication Stone.

Barry has also written a regular column covering the full spectrum of natural stone use for Natural Stone Specialist magazine.