Commercially, any calcareous, dolomitic and serpentine-based dimensional stone that can be polished is classified as marble. Its petrologic classification is much narrower, however.
True marble is a metamorphic calcareous rock, which means that – unlike sedimentary rock – its current form is not its original one. Simply put- marble was originally limestone that underwent a transformation (metamorphosis) by which, mostly through high pressure and elevated temperature, it changed into a rock physically different (though chemically the same) and it became hard enough and crystallized enough to be polished. While the overall formation of the stone may have taken 60 to 250 million years, the length of the metamorphic process is much shorter (10 million years average). Its chemical composition is still the same as the original limestone, with calcium carbonate or dolomite as the predominant components. In many instances, however, the metamorphic process brought about more than just a physical transformation of the crystalline structure of the stone. In fact, we have to take into consideration that the metamorphosis most likely began with an earthquake of apocalyptic proportions by which the orderly sediments of limestone were projected up high, cracked in the process, mixed with other surrounding minerals, and then sunk deep into the earth to “bake”.
Commercially – which is what counts in real life terms – the definition of marble is much broader. In fact, most compact limestones are also classified as marble. They actually represent the majority of the mercantile marbles available. The geological difference is represented by the fact that compact limestones are still sedimentary rocks (rather than metamorphic ones) which means that their crystallization happened somehow during the sedimentation process itself and not because of a transformation. Only a knowledgeable expert can tell the two types of stone apart. Generally speaking, most compact limestones are a little harder than marble, have smaller crystals and, because of that, are less absorbent, and polish to a deeper gloss. In the USA, true marble is found in large deposits in Alabama, Georgia, Tennessee and Vermont and, from a mechanical and grading point of view, they are among the best marbles the world over. Colorado has some really good marble, too. Compact limestones are present a little bit all throughout the Appalachian Mountains, though not quarried intensively.
There is another class of stone which is commercially classified as marble but technically it isn’t and that is green marble. Green marble has nothing in common with any other marble, besides the way it looks. In fact, there is hardly any calcite in most green marbles. Most green marbles we know of are mainly composed of minerals of the silicate group, namely magnesium silicate. Such minerals are scientifically classified as Serpentine.
There are some other green marbles (a minority, indeed) that are classified as ophicalcite. Such types of rock are of a sedimentary origination and are not only just green. For instance, the Italian Rosso Levanto is a dark burgundy stone because of the presence of high percentage of hematite in it. Regardless, green ophicalcite is a green marble which contains a small percentage of calcite.
Serpentine rocks, however, represent the vast majority of green marbles and are classified as serpentine. Very popular, and by far the best of them all (from a mechanical point of view) is the Verde Antique from Vermont, USA. These materials are rather porous; they can be 1.5 times as hard as marble and are usually resistant to acids (with the exception of ophicalcite, which is affected by many acids).
Another sedimentary stone that can be (and often times is) classified as marble is travertine. The crystallization of this stone happened during its sedimentation and its crystals are small, though not as small as in certain popular compact limestone. The main difference between most (compact) limestone and travertine is that while limestone is of an organic origin, travertine is a chemical sedimentary rock (inorganic)
Most travertines derive from evaporation of spring water rich in calcium carbonate, both in rivers near waterfalls and caves. Travertines are mostly composed of calcite or aragonite with impurities of hydrated iron oxide (loosely called Limonite). They often show fossils and imprints, especially of water plants. Besides the noted Italian travertine (Romano, Navona, Silver, etc.) vast deposits are available in Wyoming, the region of the Great Basin around Nevada and in New Mexico.
Now, coming again down to earth, let’s see the pros and cons of marble in its broader definition. Besides its indisputable and unmatchable beauty, marble is harder then, and not even close to being as porous as most limestone (with the exception of green marbles). As a matter of fact, marble in general could be defined as the least absorbent of all dimensional stones (again, green marble excluded). As a calcareous rock, marble is just as sensitive to harsh chemicals (especially acids) as limestone.
This sensitivity is more apparent when the surface of the stone is polished to a high gloss. In fact, the marks of corrosion (etches) produced by the acids are very obvious on a glossy surface, since they appear in the form of dull spots, rings, streaks, splatters, or whatever the shape of the spill was. Many wrongly refer to those etch marks as “water stains”, thus triggering the employment of massive doses of penetrating sealers which soon turn out to be totally useless for the purpose of preventing them. In fact, penetrating sealers (impregnators) can help prevent true staining, which is a discoloration of the stone due to the absorption of alien liquids of a different color, or of any oily nature, but they are obviously helpless in preventing a surface damage produced by an acid (which a stain is not, no matter how it looks).
So, it is advised to keep polished marble away from the kitchen! With the exception, of course, of green marbles of the serpentine group which, because of their totally different chemistry, are mostly resistant to acids therefore, will not show etch marks. For all other applications where wear & tear is not a predominant factor, the use of polished marble is quite all right. Just beware of popular generic cleaning products. In most cases their chemistry turns out to be too harsh and could produce the same kind of damage as accidental spills from food and beverages.
Only specialty cleaning products specifically designed to deal with the delicate chemistry of the stone should be used.