Natural stone and trade names

“Natural stone” (or dimension stone) is the trade name for stone quarried in blocks, then cut and polished for buildings, floors, worktops and monuments. The stone trade roughly divides these stones into two big families. “Marble” means polishable, calcium carbonate stones that are relatively soft: besides true marble, limestone, travertine, banded calcite (“onyx marble”) and green serpentinite are sold as marble. “Granite” means hard, polishable silicate stones: besides true granite, gabbro, diorite, syenite and gneiss are sold as granite. In geology, by contrast, marble is only the rock formed when limestone is transformed by heat and pressure, and granite is a particular plutonic rock made of quartz and feldspar.

Facades and interiors: Building facades, floors, stairs, wall cladding and bathrooms are faced with natural stone. Worktops: Kitchen and bathroom worktops are mostly made from the granite family, which resists acids and scratches; marble worktops can be dulled by lemon juice and vinegar. Monuments and sculpture: Marble is common in sculpture and monuments, granite in gravestones and memorials. Landscaping: Hard stones such as granite, andesite and basalt are used for paving, kerbs, steps and garden paving.

Steel knife or nail test: the calcite stones of the “marble” family are scratched by steel; the silicate stones of the “granite” family are not. Vinegar or lemon test (on a hidden corner): calcium carbonate stones fizz and lose their polish; granite, gabbro and gneiss are unaffected. Look closely: true marble is made of sparkling, sugary calcite crystals; limestone shows fossils, travertine small cavities and layers. Granite shows a speckled pattern of glassy quartz, opaque feldspar and shiny mica; the gabbro sold as “black granite” is dark and evenly grained, with no quartz. Marble- or granite-look porcelain slabs, laminates printed with a marble pattern and the artificial stone known as quartz countertops are not natural stone; their patterns repeat and their cut edges look different.

On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. The value of natural stone depends on the size and soundness of the blocks that can be quarried, the evenness or striking quality of colour and pattern, how well it takes a polish, its durability and the fame of the quarry. Rare and sought-after colours, and the ability to cut many matching slabs from the same stone, raise its value. Because much of the quarried stone cannot be cut into blocks and ends up as crushed stone or waste, producing good blocks takes effort and money. Marble blocks sit on the fourth step of the value ladder.

The dust from cutting and grinding natural stone contains respirable crystalline silica in quartz-bearing stones such as granite, gneiss, quartzite and sandstone, and can cause silicosis; stone workers should cut wet and wear a suitable dust mask. Concerns that granite worktops give off dangerous levels of radiation at home, on the other hand, are not supported by measurements.