We all encounter limestone buildings and marble statues during our lives. But the scientific and commercial definitions of these two rocks don't match. When geologists enter the stone dealer's showroom, and when lay people go out in the field, each has to learn a new set of concepts for these two different names.
Limestone and marble are both limerocks, an old-fashioned industrial term for stone that is roasted to produce lime, or calcium oxide. Lime is a basic ingredient in cement and much else. (For more about lime, see About Cement and Concrete.) Cement makers look at limerock as chemical feedstock of greater or lesser purity and expense. Beyond that, they are indifferent to what geologists or stone dealers call it. The key mineral in limerock is calcite, or calcium carbonate (CaCO3). Any other mineral is undesirable, but a particularly bad one is dolomite (CaMg(CO3)2), which interferes with lime manufacture.
In the past, quarriers, builders, craftsmen and manufacturers called limerock used for industrial purposes limestone. That's how limestone got its name in the first place. Limerock suitable for structural and decorative purposes, like buildings and statuary, was called marble. The word comes from ancient Greek with the root meaning of strong stone. Those historic categories are relevant to today's commercial categories.
Commercial Limestone and Marble
Dealers in stone use "limestone" and "marble" to denote a category of stone that is softer than commercial granite (or basalt or sandstone) but does not split like slate. Commercial marble is more compact than commercial limestone, and it takes a good polish.
In commercial use, these definitions aren't limited to rocks made of calcite; dolomite rock is just as good. In fact, serpentinite too has minerals softer than granite and is considered a commercial marble under the names serpentine marble, green marble or verd antique.
Commercial limestone has more pore space than commercial marble and does not wear as well. This makes it suitable for less demanding applications like walls and columns and patios. It may have some flat layering, but generally it has a plain appearance. It may be honed or polished smooth, but it is limited to a matte or satiny finish.
Commercial marble is denser than commercial limestone, and it's preferred for floors, doorways and steps. Light penetrates farther into it, giving marble a glowing translucency. It also commonly has attractive swirling patterns of light and dark, although pure white marble is also prized for statues, gravestones and decorative features. To add a bit of confusion, marble used to be called "crystalline limestone" in previous centuries. Its key feature is the ability to take a high finish.
None of these categories mean what they mean to geologists.
Geologic Limestone and Marble
Geologists are careful to distinguish limestone from dolomite rock, classifying both of these carbonate rocks as sedimentary rocks. But with metamorphism both become marble, a metamorphic rock in which all the original mineral grains have been recrystallized.
Limestone is not made of sediment derived from rocks, but instead generally consists of the calcite skeletons of microscopic organisms that lived in shallow seas. In some places it's formed of tiny round grains called ooids, formed as calcite precipitates directly from seawater onto a seed particle. The warm seas around the islands of the Bahamas are an example of an area where limestone is forming today.
Under gentle conditions underground that are not well understood, magnesium-bearing fluids may alter the calcite in limestone to dolomite. With deeper burial and higher pressure, dolomite rock and limestone both recrystallize into marble, wiping out any fossils or other traces of the original sedimentary environment.
Which of these are the real limestone and marble? I'm prejudiced in favor of geologists, but builders and carvers and lime makers have many centuries of history on their side. Just be careful about how you use these rock names.