Verde is Italian for green but Verde stone can contain a form of asbestos that leads some to believe the stone is anything but green in the environmentally friendly sense. Barry Hunt continues his examination of building stones by taking a closer look at this enigmatic material.
Is ‘Verde’ really just stone that is green? Just about any stone that is green in colour these days is immediately tagged with the description of being ‘Verde’, but this was not always the case. The term ‘Verde’ used to be reserved for those stones that were relatively soft and deep green; stones that geologists classify as ‘serpentinites’.
Yes, yet another scientific word but a relatively familiar one. It describes a rock that might be considered to fit the descriptive terms given to it in the dictionary, such as ‘complicated’, ‘intricate’, ‘elaborate’ and ‘knotty’. But it is actually the mineral that forms the bulk of the rock that gives it its name – serpentine.
It got its name because it was felt the stone resembled the reptilian scales of a snake or lizard. One type of serpentine is actually called ‘lizardite’, although this is purely coincidental as it was named after the location where it was first identified – the Lizard Peninsula in Cornwall (which, in turn, is believed to have gained its name as a corruption of the Cornish for ‘High Court’, Lys Ardh).
But there is no need to over-complicate matters. Suffice it to say there is a considerable range of minerals classified within the serpentine group, although most serpentinites used as building stone predominantly comprise lizardite. And lizardite is one form of the mineral chrysotile, which has associations with the asbestos trade… but more of this later.
There are also many non-serpentine minerals associated with serpentinite rocks, the best known being calcite and dolomite, the common carbonates that typically form the white veins that cut randomly through the stone. Minerals such as chlorite, talc, magnesite and brucite are also often present.
Serpentinites are traditionally referred to in the stone trade as being marbles because they are relatively soft and can take a polish. However, they are not marbles because they do not exhibit sufficient metamorphism from heat and pressure effects to be classed as such (see NSS August 2013 for my discussion of marble).
The changes in the rock are the result of low heat and pressure, along with plenty of water to hydrate the new minerals being formed. The parent rock is usually basaltic, which means low in silica, and there are typically high proportions of minerals such as olivine and pyroxene, which are referred to as ‘mafic’ minerals. Geologists (as ever) have a term for such rocks: ‘ultramafic’. It is the mafic minerals that readily decay into serpentine minerals under the right conditions.
Serpentinites commonly form from chunks of oceanic crust that have been sucked down into the earth only to float back up to the surface again. The most apt geological term used to describe serpentinites is ‘greenschist’.
This is easier to decipher as it really does mean schist that is green in colour (schist being a medium-grained metamorphic rock with evidence of thin banding). That is now too many unusual and some might say meaningless geological words for one article, so let’s get on with how Verde is used as building stone.
Origin and Historical Use
The mode of formation combined with the properties of Verde stone mean that quality supplies are particularly limited compared with other stone types.
There is also the issue of why such a typically weak and potentially poorly performing stone should be used when there are many better and usually cheaper alternatives that are simpler to use.
The principal reason, unsurprisingly, appears to be the colour, affirming the rationale as to why ‘Verde’ became a stone grade. The other reason is the relative softness of Verde, which is classed at around 2.5 on Mohs’ scale of hardness. That is harder than gypsum but softer than calcite, which makes Verde good for sculpture. But really there are few reasonable grounds to use it for most other masonry purposes.
With increasing carbonate content the hardness also increases, and those Verdes containing dolomite reach up to 4 on Mohs’ scale. There are serpentinites that are even harder, but often they are not green nor used for building stone.
Verde stones are most commonly used internally and usually only in small quantities as part of an ornate colour scheme or for decorative constructions.
Verde stone often turns up in stone inlay or marquetry within furniture, especially ornate tables. There is not much Verde used externally in the UK as it does not react well in our climate, so you have to go further afield to typically warmer and drier climes to find locations where serpentinite has survived outside. There was a collection of 30 sculptures carved from the local serpentinite in Gandhara, north-west Pakistan, that are believed to date from between the 1st and 6th Centuries AD, that were brought ot the UK and left exposed at Antony House in Cornwall. They suffered considerable decay.
Verde suffers decay to minerals within it such as talc, resulting in disaggregation of the constituents that eventually leaves nothing but a pile of fine dust. Iron compounds may also be released and many weathered serpentinites will appear blood red.
There is almost little point in polishing Verde to be used externally as the fineness of the serpentine crystals leads to exfoliation and lightening in colour, with different grades of the stone becoming evident in a short space of time.
Weathering Verde develops a greasy feel, due to the softness and fineness of the minerals, much like soapstone – and, indeed, the two stones have many similar minerals.
It is considered that those Verdes with higher proportions of carbonate minerals (that have possibly even taken over from the serpentine as the dominant constituents of the stone) are those that provide the highest strength and better durability. The Rajasthan Green (Raj Green) stone from India is a good example of this.
These days, most commercially available serpentinite originates from China, Guatemala, India, Italy, Pakistan, Portugal, Spain and the United States.
In the UK, significant serpentinites are found in Cornwall but are extremely limited in use. The Polyphant stone is still quarried for statuary and other ornamental work and it has been used locally in the past for building purposes.
Mona Marble found on the Isle of Anglesey is another Verde. It was principally used for decorative furniture, statuary and monuments, and is believed to have been used for some of Napoleon’s furniture while he was in exile on Saint Helena. The Connemara marble from Ireland also contains serpentine minerals but is not a serpentinite.
Further afield, ‘Lapis Atracius’ that the Romans formerly used is now known as Verde Antique (or ‘Antico’). This is an unusual Verde as it is a serpentinite breccia, or basically lumps of broken up serpentinite within a fine matrix. The stone is quarried near Larissa in Thessalia, Greece, where the ancient town of Atrax is believed to have stood and which provides the name for this stone. The Romans held Lapis Atracius in great esteem for its beauty and rarity.
The Basilica of San Giovanni in Laterano contains 24 columns within the niches of the 12 apostles. Those ornamenting the high altar of San Agnese in Piazza Navona in Rome are described as magnificent. Pillars in the Hagia Sophia in Istanbul are also believed to have been constructed of this stone.
The United States is blessed with Verde on both sides of the continental divide. It is a major resource found in Vermont that has been continuously mined for the past 100 years. Chris Curtis has used the ‘Vermont Verde Antique’ in his sculptural work to great effect. In 1965, California became the first state to have an officially designated state rock when it decided to bestow the title on its serpentinite, although recently there have been calls to strip it of its title because of California’s drive to be seen as the ecological state. Serpentinite is associated with asbestos, although that association is actually not justified in this particular instance. In Chester County, Pennsylvania, a number of pre-revolutionary buildings were built using blocks of a local serpentinite, although few of these survive and the use of the stone for building quickly ceased when it was found to suffer such rapid deterioration.
An 1880 census in Philadelphia listed six significant buildings with serpentinite façades using materials from Chester County. They included the 19th Street Baptist Church that survives but continues to suffer aggressive decay.
The town of Bhera in the Indian Punjab was famous for working a relatively pure form of green serpentine obtained from quarries in Afghanistan into lapidary work, cups, ornamental sword hilts, and dagger handles. This high-grade serpentine ore was known as ‘sang-i-yashm’ or ‘false jade’. It is easily carved, takes a good polish and is said to have a pleasingly greasy feel.
Properties and Current Use
In the UK, the principal use of Verde remains decorative. It is often used for fireplace surrounds or as small inserts in floors of other stones (whole floors of Verde are relatively rare). When it has been used as cladding it has been mostly for interiors, although there is the occasional rush of blood to someone’s head and Verde is used outside, usually in minimal amounts for ground level shop fronts, for example. The stone inevitably looks tired after a short time.
When Verde is cut into slabs, typically the panels created are reinforced with resin and mesh to provide additional strength, as this stone is notorious for suffering splitting due to the veining and general weakness.
Table 1 includes some typical values for Verde, although it must be considered extremely general given the extreme range in performance exhibited by Verdes.
In the world’s hotter and more arid climes, where it has a much increased chance of survival, Verde is sometimes used for external cladding but mostly the rest of the world uses in much the same way as it is used in the UK.
The single greatest concern with Verde has been that association with asbestos that I said I would return to. There is a lot of misinformation about this. An extremely strong lobby rightly ensures awareness of asbestos remains high in the public psyche but it can cause unwarranted panic.
The concern with Verde comes from chrysotile, a serpentine mineral that takes on a needle-like crystal form regarded as asbestiform. All the other asbestiform minerals are types of amphibole, and thus are not considered further here.
Typically, chrysotile is not found in Verde stones used for building purposes because the stone would be seriously weakened by the presence of such mineral forms in significant proportions.
If chrysotile were found to be present in a building stone it would be locked into the stone and of no significant threat. However, if someone started cutting or grinding the stone and producing dust, then this would be potentially respirable and thus a potential threat to health. There is insufficient research available on this and it is always better to be safe than sorry.
Stone Federation Great Britain’s advice on the presence of asbestiform minerals in Verde stone suggests that it is prudent to establish whether fibres in the respirable range are generated during cutting, grinding or polishing operations, particularly if the work is carried out dry.
The Asbestos (Prohibitions) Regulations have banned the import, supply and use of blue, brown and white asbestos (crocidolite, amosite and chrysotile, respectively) when they are part of an asbestos admixture product. Natural stones such as Verde that may contain chrysotile would not be classed as asbestos admixture products and so their import and use is not currently restricted.
Conclusion
Verde stone will always be a bit part player in the natural stone industry. You could say it exhibits the greatest desirability of form and structure of any natural stone, but this comes at huge cost, almost making it unusable. More than any other stone, the properties of Verde need to be carefully assessed and matched to the environment in which the stone will be used. For the most part, the best advice is to leave it be and keep it away from water.
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 also written a regular column covering the full spectrum of natural stone use for Natural Stone Specialist magazine.