Limestone is not a marble, but marble is a limestone. Limestones are made up of calcite from shells, coral and other debris. They are what we call sedimentary rocks—that is, they have been formed by the breakdown of other rocks, shells, etc. Marbles are limestones that have been subjected to great heat and pressure and as a result, have changed (metamorphosed) into marble.
There are several differences, and they are very important. Limestones are usually loosely held together, and may differ in porosity from marble. The coarse grain of some limestones give them excellent wearability. Limestones can contain numerous fossil impressions that are well preserved. If a piece of stone has various shell or animal-like patterns there’s a very good chance that it’s limestone. Limestones come in various colors, but most are shades of brown or tan, with some leaning toward gray and red. All seem to fall into the earth-tone color range. Limestone is becoming increasingly popular in the American West and Southwest.