What is and what is not natural marble?
Marble is a material that has been integrated into architecture since ancient times. We all know examples of great architectural works made of marble and associate marble with luxury, large public spaces, and elegance. We all know what marble is, or do we?
Things are not so clear when we start to think about it a little more. Are all the stones traditionally used in architecture marbles? What about granites and quartzites? Are stones that are not polished marbles? What colors can marble be?
The fact is that there are two meanings of marble. One is commercial and industrial. The other is more purely geological. The first one encompasses a wide variety of ornamental rocks that, although strictly speaking, would not be marbles from a geological point of view, fit perfectly into the concept of marble used in architecture and interior design. So, marble is any industrial rock, mainly carbonate, that can be polished without the need for chemical procedures and/or additives. This is the definition of the Marble Institute of America (MIA). This is the definition that fits the international standard ASTM C119, which regulates terminology for dimension stone, which is stone that has undergone a selection and cutting process to achieve certain measurements. We will have the opportunity to talk about measurements and cutting techniques in other entries of this blog.
Thus, there will be rocks that, while technically sandstones, can be commercially considered "marbles" because they exhibit the characteristics of marbles for their industrial exploitation. Their colors and hardness will vary, as in the case of other marbles, depending on the "impurities" that have played a part in their formation. Thus, rocks with a high content of iron oxides will tend to be reddish. Diopside will give green shades. There are usually graphite insertions, with their characteristic dark gray hue. In the case of marbles derived from sandstones, grainy patterns often appear, as if made with charcoal, different from classic marbles, but also beautiful.
So, what the hell is marble? Marble, properly speaking, from a geological point of view, is a metamorphic rock formed when limestone is subjected to high temperatures and pressures and reaches a high degree of mineralization. It is composed mostly of calcite (CaCO3), although it may contain traces of other minerals. It has a hardness of 3-4 on the Mohs scale and a resistance between 40 and 100 MPa.
When the metamorphic process occurs not on calcite, but on dolomite - CaMg(CO3)2 - the result is a dolomitic marble, slightly denser and harder. And this would be the difference between "authentic" marble and the rest of decorative stones known as marbles: crystallization occurs in calcite or dolomite (calcium and magnesium carbonate).
In general, these "authentic" marbles would present larger crystals, with an appearance of crystallized sugar, and would have an absence of fossils. But for their use in construction, it is not important that an ornamental stone fits into the purely geological definition of marble. That being said, it should also be noted that many of the most precious and well-known marbles since ancient times are also marbles according to this latter definition.