Limestones are made of calcite from shells, coral and other debris. They are known as sedimentary rocks, because they are formed by the breakdown of other rocks, shells, etc. Limestones are usually loosely held together and may differ in porosity from marble. The course grain of some limestones give it excellent durability. Limestone can have numerous fossil impressions that are well preserved. If a piece of stone has various shell or animal like impressions in it, there is a good chance it is limestone. To identify limestone, look for fossil like impressions and loose grain structure. Limestone comes in various colors but most are shades of brown or tan, some leaning towards gray and red. All shades seem to fall into the earth tone color range. Limestone is becoming increasingly popular in the West and Southwest.