WHAT ARE THE SOFTER NATURAL STONES AND WHERE CAN THEY BE USED? landscape stone

Why are some natural stones considered to be soft when they all appear to be hard? The answer lies within ‘relative’ hardness. Mohs scale of hardness was invented in 1812 and compares the relative hardness of ten minerals. Diamond is the hardest and rates a 10, while granite is the toughest natural stone at 6. Limestone comes in at 3 as does its metamorphic counterpart, marble. Softer stone is easier to dress or carve but does not wear or weather quite as well as harder stone. Here we discuss some of the more popular soft stones along with suitable applications.

Irregular stones

Sedimentary rock

Limestone, sandstone and shale are the most common types of sedimentary rock. These were formed through tremendous pressure, over millions of years, bearing down on sediment that had fallen to the ocean floor.

Slate

The layers in slate are described as being “foliated” and they are easily split apart to create whatever thickness is required. UK slate is considered to be hard and was traditionally used as roofing, while soft slate is found in China, Spain, Italy and the USA. With a broad range of natural slate colours, a multiple array of designs can be achieved, from contemporary to classic, rustic to refined. Slate is often recommended for high traffic areas, thanks to its remarkably durable composition. It is also non-porous and does not easily react with acid liquids. It is fire proof, weather resistant and achieves good slip resistance dues to its riven finish.

Limestone

Limestone is a very common building material and is formed mainly from the mineral calcite, derived from the calcium in bones and seashells deposited over the millennia and forced together through pressure. While it also contains magnesium, it is harder and more weather resistant, and can also be polished. Portland stone from the eponymous island in Dorset is probably the single most famous type of limestone and was used to build many of London’s great buildings. It is used for external cladding as well as paving, fireplaces and other internal and external decorative features. Its soft colours are its trademark visual attributes.

Sandstone

Sandstone was probably the most commonly used building stone prior to 1800, for everything from bridges to stately buildings. As may be inferred from its name, it is formed when sand, organic matter, calcite and a variety of other minerals were fused together under incredible pressure over the millennia. Available with either a coarse or fine texture and traditionally supplied in a matt finish. Primarily cream, red or grey in the UK, its colour depends on the additional minerals contained within it. Silica imparts whiteness, whilst iron will give a reddish-brownish tint. Its main areas of application are walls and flooring, or exterior paving.

Marble

Marble is a derivative of limestone, formed through the metamorphosis of colossal heat and pressure over millions of years. Although relatively soft compared to other stones, marble tends to polish incredibly well. Traditionally marble is used in doors and helps to create a high-end finish.